(54) باب إِقَامَةِ الصُّلْبِ فِي السُّجُودِ
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.'"
Reference: Sunan an-Nasa'i 1111
In-book reference: Book 12, Hadith 83
English translation: Vol. 2, Book 12, Hadith 1112