The slave of Allah is subsequently prescribed to say “Allah Akbar” “Allah is the Most Great” and then fall down in prostration to the floor to let his limbs individually demonstrate their servitude to Allah; his forehead placed on the floor before his Lord, his face – the most honourable part of his body – comes together with the dust on the floor, and all the while his heart humbles itself to Allah. The arrangement of this posture is such that his lower body ends up in a higher level than his upper limbs (i.e. the face), emphasising his submission. His heart and body are thus rendered humble and submissive before the Greatness and Glory of the Lord. Moreover, when the state of the heart conforms to the state of the body in this position (i.e. both are humble, submissive and down to earth) it is as if the heart too prostrates to Allah along with the forehead, nose, face, hands, knees and feet. A slave with such humility and submissiveness shall be among Allah’s favoured servants. This is because the closest a slave comes to his Lord is when he is in the state of prostration.
In this posture, it is prescribed for the slave to not have any part of his body depend on any other part for support; that is, he should avoid placing the weight of his thighs on whistles, maintain space between his belly and his legs, and keep a space between his arms by spreading them to the sides away from his body. This is so that each part office physical self can individually demonstrate its slavery to Allah. And it follows from what has been said that the slave (of Allah) should in this posture be at the closest point to his Lord, as the Prophet, peace and blessings of Allah be upon him, said:
“The closest a slave is to his Lord is when he is prostrating.” (Recorded in Muslim)
The reality of the prostration of the heart is its submission to Allah, and the heart is able to remain in prostration until the Day of Judgement. On of the righteous Salaf was asked: “Does the heart prostrate?” He answered: “Indeed it does, by Allah it prostrates such that it does not raise its head even once, and remains in prostration until it meets Allah, Exalted be He.” [Shaykh al-Islam Ibn Taymiyyah attributed this statement to Sahl Ibn Abdullah al-Tustari]
Inner Dimensions of the Prayer by Ibn Qayyim Al-Jawziyyah Pages 50-51