Could you please tell me the supplication that the one who is staying home should offer for the one who is travelling, and vice versa? Should we say this supplication in front of them out loud, or say it quietly to ourselves?
Firstly:
It is mustahabb for the one who is bidding farewell to a friend of his who is going to travel to say to him: “Astawdi‘ Ullaha deenaka wa amaanataka wa khawaateema ‘amalika (I leave your religious commitment in the care of Allah, as well as what you leave behind [of family and wealth], and the last of your deeds).” That is because of the report that is proven from the Prophet (blessings and peace of Allah be upon him), as narrated in Sunan Abi Dawood (2601) and elsewhere, from ‘Abdullah al-Khatmi, who said: When the Prophet (blessings and peace of Allah be upon him) wanted to bid farewell to the army, he said: “Astawdi‘ Ullaha deenakum wa amaanatakum wa khawaateema a‘maalikum (I leave your religious commitment in the care of Allah, as well as what you leave behind [of family and wealth], and the last of your deeds).”
Classed as saheeh by Shaykh al-Albaani.
It was narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) bade me farewell and said: “Astawdi‘uka Allaha alladhi laa tadee‘u wadaa’i‘uhu (I leave you in the care of Allah, in Whose care nothing is lost).”
Narrated by Ibn Maajah (2825) and others; classed as saheeh by al-Albaani.
It was narrated that Anas (may Allah be pleased with him) said: A man came to the Prophet (blessings and peace of Allah be upon him) and said: O Messenger of Allah, I want to travel, so give me some provision for my journey. He said: “Zawwadaka Allahu at-taqwa (May Allah supply you with piety).” He said: Give me more. He said: “Wa ghafara dhanbaka (may He forgive your sin).” He said: Give me more, may my father and mother be sacrificed for you. He said: “Wa yassara laka al-khayra haythuma kunta (May He make good easy for you wherever you may be).”
Narrated by at-Tirmidhi (3444); classed as saheeh by Shaykh al-Albaani.
It was narrated from Saalim that Ibn ‘Umar used to say to a man, when he wanted to travel: Come close to me, and I will bid farewell to you as the Messenger of Allah (blessings and peace of Allah be upon him) used to bid farewell to us. Then he would say: “Astawdi‘ Ullaha deenaka wa amaanataka wa khawaateema ‘amalika (I leave your religious commitment in the care of Allah, as well as what you leave behind [of family and wealth], and the last of your deeds).”.
Narrated and classed as saheeh by at-Tirmidhi (3443); also classed as saheeh by al-Albaani.
These are the supplications that the one who is staying home may offer for the one who is travelling.
With regard to the supplication of the traveller for the one who is staying home, what we have found concerning that is the report narrated by Ibn as-Sunni in ‘Aml al-‘Yawm wa’l-Laylah (516), from Moosaa ibn Wardaan: Abu Hurayrah said: Shall I not teach you some words that the Messenger of Allah (blessings and peace of Allah be upon him) taught me?
When you want to travel, or leave a place, say to your family: “Astawdi‘ukum Allaha alladhi laa yukhayyibu wadaa’i‘ahu (I leave you in the care of Allah, Who does not neglect what is entrusted to Him).”
Its isnaad includes Ibn Luhay‘ah, who is da‘eef (weak) and had a poor memory.
It was classed as hasan by Shaykh al-Albaani (may Allah have mercy on him) in as-Saheehah (16). See also: as-Silsilah ad-Da‘eefah (1470).
If he says to the one who is bidding farewell to him: Jazaak Allahu khayran (may Allah reward you with good), or he offers supplication for good for him, that is good and is something prescribed, in sha Allah.
An-Nawawi (may Allah have mercy on him) said: It is mustahabb for him to bid farewell to his family, relatives, friends and neighbours, and to ask them to offer supplication for him, and for him to offer supplication for them.
End quote from al-Adhkaar (217).
Secondly:
With regard to raising the voice when offering this supplication, that should be in accordance with what is customary, so that his friend can hear him, because the hadiths quoted above indicate that the Prophet (blessings and peace of Allah be upon him) used to make those to whom he was bidding farewell hear his supplication for them. Ibn ‘Umar and other companions of the Prophet (blessings and peace of Allah be upon him) used to do likewise.
And Allah knows best.