Du’a in Prostration
The Messenger of Allah (salAllahu ‘alayhi wa sallam) said:
The closest a servant is to his Lord is when he is in prostration. So increase your supplications therein! (Muslim, 482)
Shaikh Ibn al-'Uthaymeen (rahimahullah) stated:
This is because he has placed his forehead (face) — the highest and most honourable part of his body — down on the ground where people tread upon with their feet. In doing so he demonstrates the perfection of his subservience to Allah.
The one standing up is actually higher up physically than the one down in prostration, however since the prostrating one humbled himself to Allah, consequently Allah raised him and so he is in reality closer to Allah (in this position).
So it is appropriate you make plentiful supplication in these prostrations — whether obligatory or voluntary (nafl/sunnah) prayers — and you supplicate for whatever you wish (after completing the prescribed supplications) as long as it not sin or cutting the ties of kinship.
So a student of knowledge may ask "O Allah increase me in knowledge and grant me understanding and memorisation",
the one building a house may ask "O Allah aid me in the completion of the house",
the youth may ask "O Allah grant me a (righteous) spouse"… and so on
[Sifaatus Salaah by Shaykh Ibn al-Uthaymeen (rahimahullah), pg. 124]
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