“The Closest a Servant is to his Lord is when he is in Prostration”[Muslim 482]
Shaikh Ibn ul-‘Uthaymeen رحمه الله stated:
“This is because he has placed his forehead (face) – the highest and most honourable part of his body – down in the ground where people treat upon with their feet, in doing so he demonstrates the perfection of 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 actually closer to Allah.
So it is appropriate you make plentiful supplications in these prostrations – whether obligatory or supererogatory 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 memorization”, 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 wife”…and so on”.
صفة الصلاة (بتصرف يسير) الشيخ العثيمين ص124