Allah's Messenger said: "The Salat is not acceptable if a man is not at rest - meaning his back - while bowing and prostrating."
Jami` at-Tirmidhi Book 2, Hadith 117
← Previous Hadith "The Prophet prohibited wearing Qassi, and that which is dyed with safflower, and from the gold ring, and from reciting the Quran while bowing."

Jami` at-Tirmidhi Book 2, Hadith 116 Next Hadith → "When Allah's Messenger would raise his head from bowing he would say: (Sami Allahi liman hamidal. Rabba na lakal-hamd, mil'as-samawati wa mil'al-ardi wa mil'a ma bainahuma wa mil'a ma shi'ia min shay'in ba'd) 'Allah listens to those who praise Him. O our Lord! And to You is the praise that fills the heavens and fills the earth, and fills what is between them, and fills whatever there is beyond that You will.' He said: There are narrations on this topic from Ibn Umar, Ibn Abbas, Ibn Abi Awfa, Abu Juhaifa, and Abu Sa'eed.

Jami` at-Tirmidhi Book 2, Hadith 118