´It was narrated that Abu Mas'ud said:` "The Messenger of Allah (ﷺ) said: 'No prayer is valid in which a man does not maintain his back (at ease) when bowing and prostrating.'"
Hadith ReferenceSunan an-Nasa'i / The Book of The At-Tatbiq (Clasping One\'s Hands Together) / Hadith: 1112