Abu Hurairah (RA) narrated that Allah’s Messenger (SAW), said: ‘The nearest a slave can be to his Lord is when he is prostrating, so invoke (supplicate) Allah (SWT) much in it. [Muslim, abu Dawud, an-Nasa’i and others, Sahih al-Jami #1175]
When a Muslim is in his Salat (prayer) he is facing Allah (SWT) and when he prostrates he is the nearest he can be to Allah (SWT) so it is best to invoke Allah (SWT) at this time. It is said that while in prostration, one should not ask for worldy needs but for the Hereafter.
"When Musa traveled to meet with Khadir, he got tired and hungry on the journey, and said to his servant, "Bring me my dinner, for this journey has made us very tired" [Kahf: 62]. For in this journey, he was traveling to another created being.
But then, when his Lord promised him a meeting for thirty full days, and He blessed him with ten more to complete forty, on Mt. Sinai, he did not eat during this meeting, nor did he feel hungry or tired. For in that journey, he was traveling to his Lord.
And so from this we learn that when the heart is on its journey to its Lord, it shall not find any pain, or tiredness, or suffering, yet when it is on a journey to another created being, it shall suffer from all of these."
Muhammad is the Messenger of Allah ; and those with him are forceful against the disbelievers, merciful among themselves. You see them bowing and prostrating [in prayer], seeking bounty from Allah and [His] pleasure. Their mark is on their faces from the trace of prostration. That is their description in the Torah. And their description in the Gospel is as a plant which produces its offshoots and strengthens them so they grow firm and stand upon their stalks, delighting the sowers - so that Allah may enrage by them the disbelievers. Allah has promised those who believe and do righteous deeds among them forgiveness and a great reward.[48:29]