Fard (obligatory) prayer takes precedence over nafl (supererogatory) prayer, carries more weight, and it is greater in reward

Firstly:

Omitting the prayer is one of the gravest of major sins. Whoever omits a prayer deliberately – even a single obligatory prayer – has exposed himself to the wrath, anger and punishment of Allah. In fact, some scholars are of the view that the one who omits a single prayer, with no excuse, until the time for it has ended, is a disbeliever.

See questions no. 39818 and 47123.

Ibn al-Qayyim (may Allah have mercy on him) said:

There is no difference of opinion among the Muslims that omitting an obligatory prayer deliberately is one of the worst and gravest of major sins. The sin of that is greater than the sin of murder or seizing wealth unlawfully, and greater than the sin of zina, theft or drinking alcohol. The one who does that is exposed to the punishment and wrath of Allah, and to disgrace in this world and the hereafter. End quote.

As-Salaah wa Ahkaam Taarikiha (p. 31).

Secondly:

We do not know of any hadith from the Prophet (blessings and peace of Allah be upon him), or of any report from his companions (may Allah be pleased with them), that defines the extent or duration of the punishment in hell to which the one who does not pray exposes himself, whether he omits a single obligatory prayer or more than that.

The famous hadith that is well-known and often quoted by the common folk, which says that the one who omits a single prayer deliberately in this world and does not make it up will make it up on the floor of Hell, is baseless.

See question no. 143827.

But it is essential to note that the one who omits a single obligatory prayer is committing a grave action and is exposing himself to severe punishment. According to some of the scholars he is a disbeliever who is beyond the pale of Islam, as stated above.

Ibn Hazm (may Allah have mercy on him) said: It was narrated from ‘Umar, ‘Abd ar-Rahmaan ibn ‘Awf, Mu‘aadh ibn Jabal, Abu Hurayrah and other Sahaabah (may Allah be pleased with them) that whoever omits a single obligatory prayer deliberately, until the time for it has ended, is a disbeliever and apostate.

End quote from al-Muhalla (2/15).

The Permanent Committee, headed by Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him), issued fatwas on this basis. Fataawa al-Lajnah (6/40, 50).

See the answers to questions no. 83165 and 210371.

So the Muslim should beware of being heedless with regard to the prayer, and he should fear the punishment of Allah, no matter how long or short it may last, for if a person owned everything in the world, and as much again, he would give it to ransom himself from the touch of the fire.

Muslim (2807) narrated that Anas ibn Maalik said: The Messenger of Allah (blessings and peace of Allah be upon him) said: The most affluent of the people in this world, of those who will go to Hell, will be brought on the Day of Resurrection and dipped once in the Fire. Then it will be said: O son of Adam, did you ever see anything good? Did you ever have any pleasure? He will say: No, by Allah, O Lord. Then the most destitute of the people in this world, of those who will enter Paradise, will be brought and dipped once in Paradise, and it will be said to him: O son of Adam, did you ever see anything bad? Did you ever experience any hardship? He will say: No, by Allah, O Lord. I never saw anything bad and I never experienced any hardship.”

Whoever is heedless about the issue of prayer, if Allah decrees punishment for him on the Day of Resurrection – Allah forbid – no one knows how long he will remain subject to that punishment except Allah.

Thirdly:

No specific person can be deemed to be in hell except on the basis of a shar‘i text. So it cannot be said of any specific person who did not pray that he is in hell because of his not praying, because he may have repented.

The things that may prevent the specific punishment from befalling the sinners among those who affirm the oneness of Allah are many; they may be ten or more, as was mentioned by Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) and his student Ibn al-Qayyim, and other scholars.

Rather what we must do is enjoin what is right and forbid what is wrong, advise the Muslims sincerely, and warn against heedlessness with regard to religious obligations, rituals and rulings.

Fourthly:

The obligatory prayer takes precedence over nafl (supererogatory) prayer. Nafl prayer is Sunnah prayer, and there are confirmed Sunnah prayers (Sunnah mu’akkadah) and Sunnah prayers which are not confirmed. The confirmed Sunnah prayers include the twelve regular (rawaatib) rak‘ahs, the prayer for rain (aalaat al-istisqaa’), Taraaweeh prayer, and the eclipse prayer (salaat al-khusoof).

Sunnah prayers which are not confirmed include four rak‘ahs before ‘Asr, two rak‘ahs before Maghrib, and two rak‘ahs after doing wudoo’.

The confirmed Sunnah (Sunnah mu’akkadah) prayers are of greater virtue, are more important, bring a greater reward and carry more weight than the unconfirmed Sunnah prayers.

All the Sunnah prayers are not obligatory, so one is not sinning by omitting them.

The obligatory (fard) prayer is compulsory, and the one who omits it is sinning. It is of greater virtue, more important, brings a greater reward and carries more weight than the Sunnah prayers, both confirmed and unconfirmed.

Al-Bukhaari narrated in his Saheeh (6502) that Abu Hurayrah said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Verily Allah says: ‘Whoever shows enmity to one who is close to Me, I shall be at war with him. My slave does not draw near to Me with anything more loved by Me than the religious duties that I have enjoined upon him, and My slave continues to draw near to Me with supererogatory works so that I shall love him. When I love him, I am his hearing with which he hears, his sight with which he sees, his hand with which he strikes and his foot with which he walks. Were he to ask something of Me, I would surely give it to him, and were he to ask Me for refuge, I would surely grant him it.”

An-Nawawi (may Allah have mercy on him) said:

It is well-known that the reward for an obligatory prayer is greater than the reward for a nafl prayer, because Allah, may He be glorified and exalted, said [in the hadith qudsi quoted above]: “My slave does not draw near to Me with anything more loved by Me than the religious duties that I have enjoined upon him.” End quote.

Sharh an-Nawawi ‘ala Muslim (7/92).

Al-Haafiz (may Allah have mercy on him) said:

From this it may be understood that performing obligatory religious duties is the dearest of deeds to Allah.

At-Toofi said: The command to do obligatory duties is definite, and failing to do them leads to punishment, unlike nafl deeds, even though in both cases there will be reward (for doing the deeds). However, the obligatory duties are of a higher standing, therefore they are dearer to Allah, may He be exalted, and bring one closer to Him.

Moreover, the obligatory duties are like the root and foundation, and the nafl deeds are like the branches and structure. Doing the obligatory duties in the manner enjoined is complying with the command and showing respect and veneration to the One Who issued the command, by submitting to Him, and is a demonstration of the greatness of the Lord and the humility of the slave; therefore drawing close to Allah by means of that is the greatest of deeds. End quote.

Fath al-Baari (11/343).

But the Muslim must strive hard to regularly offer the Sunnah prayers, because they will make up for any shortfall in the obligatory prayers.

At-Tirmidhi (413) narrated that Abu Hurayrah said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: The first of his deeds for which a person will be brought to account on the Day of Resurrection will be his prayer. If it is good, then he will have succeeded and prospered, but if it is lacking, then he will have lost and failed.

If anything is lacking from his obligatory prayer, the Lord, may He be glorified and exalted, will say: Look and see, does My slave have any voluntary prayers, which could make up for what is lacking from his obligatory prayer? Then all of his deeds will be dealt with in a similar manner.

Classed as saheeh by al-Albaani in Saheeh at-Tirmidhi.

And Allah knows best.