He wants to pray the obligatory prayers only and not the naafil prayers

Observing the naafil (supererogatory) prayers and offering them regularly is one of the greatest means of earning the love of Allaah and it brings Paradise and divine mercy. Al-Bukhaari narrated (6502) that Abu Hurayrah said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said, Allah says: “Whosoever shows enmity to someone devoted to Me, I shall be at war with him. My servant draws not near to Me with anything more loved by Me than thereligious duties I have enjoined upon him, and My servant continues to draw nearto Me with supererogatory works so that I shall love him. When I love him I am hishearing with which he hears, his seeing with which he sees, his hand with whichhe strikes and his foot with which he walks. Were he to ask [something] of Me, Iwould surely give it to him, and were he to ask Me for refuge, I would surely grant him it. I do not hesitate about anything as much as I hesitate about [seizing] thesoul of My faithful servant: he hates death and I hate hurting him.”  

So the Muslim should be very ambitious and keen, and should not accept anything less than that; rather he should seek to carry out his religious duties completely and perfectly, as he is required to do with regard to his worldly duties. 

However, if a Muslim restricts himself to doing only the obligatory duties, in prayer and in other matters, and does not fall short in any of that, there is no sin on him, even though neglecting the Sunnahs in general is something blameworthy according to the scholars and Imam Ahmad said: “Whoever does not pray Witr is a bad man whose testimony should not be accepted.” 

Al-Bukhaari (46) and Muslim (110) narrated that Talhah ibn ‘Ubayd-Allaah said: A man came to the Messenger of Allaah (peace and blessings of Allaah be upon him) and when he drew close to him he asked him about Islam. The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Five prayers each day and night.” He said: “Do I have to do anything else?” He said: “No, not unless you do it voluntarily.” And the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “And fasting Ramadaan.” He said: “Do I have to do anything else?” He said: “No, not unless you do it voluntarily.” And the Messenger of Allaah (peace and blessings of Allaah be upon him) told him about zakaah. He said: “Do I have to do anything else?” He said: “No, not unless you do it voluntarily.” The man left, saying, “By Allaah, I will not do any more than this or any less.” The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “He will succeed, if he is speaking the truth.” 

Al-Nawawi said: This is to be understood as meaning that he did not offer naafil prayers and he did not omit any of the obligatory prayers. Such a person will undoubtedly be successful, even though his persisting in not offering Sunnah prayers is something blameworthy for which his testimony is to be rejected. But he is not sinning thereby, rather he will be successful and will attain salvation. And Allaah knows best.  

Sharh Muslim, 1/121 

It should be noted that the naafil prayers bring immense reward and great virtue. It was narrated that Abu Hurayrah (may Allaah be pleased with him) said: “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 sound then he will have succeeded but if it is not sound then he will be lost and doomed. If anything is lacking from his obligatory prayers, the Lord will say: ‘Look and see whether My slave has any voluntary (prayers),’ and the shortfall from his obligatory prayers will be made up from that. Then all his deeds will be dealt with in like manner.” Narrated by al-Tirmidhi, 413; Abu Dawood, 864; classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

And the Prophet (peace and blessings of Allaah be upon him) said: “Whoever prays twelve rak’ahs each day and night, a house will be built for him in Paradise because of them.” Narrated by Muslim, 728 

May Allaah help you to do your best and to say and do that which is good and true. And Allaah knows best.