Hadith: The nearest a slave is to his Lord is when he is prostrating, so increase (your) supplications (while in this state)

عن أبي هريرة رضي الله عنه عن النبي صلى الله عليه وسلم قال: «أقْرَبُ ما يَكون العبد مِنْ رَبِّهِ وهو ساجد، فَأَكْثروا الدُّعاء».
[صحيح] - [رواه مسلم]

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) said: "The nearest a slave is to his Lord is when he is prostrating, so increase (your) supplications (while in this state)."
Sahih/Authentic. - [Muslim]

Explanation

Abu Hurayrah (may Allah be pleased with him) narrated that the Messenger (may Allah's peace and blessings be upon him) said: "The nearest a slave is to his Lord is when he is prostrating." This is because when a person prostrates, he is placing his noblest body part, which is his face, on an area that is touched by feet. The face is also the highest point of his body, and he is placing it close to the lowest part of his body, which are his own feet – out of humility and devotion to Allah. This is why the closest a person is to Allah is when he is prostrating. The Messenger (may Allah's peace and blessings be upon him) ordered us to increase out supplications while prostrating so that our words of supplication would match our physical position of submission. This is why we say in our prostrations: "Glory be to Allah, My Lord, the Most High," to emphasize Allah’s greatness and power over us in His essence and his Attributes; and to acknowledge that the human is the lowest in relation to His Glory.