Sunan Ibn Majah
872
Sahih+3

It was narrated that Rashid said:“I heard Wabisah bin Ma’bad saying: ‘I saw the Messenger of Allah (ﷺ) performing prayer, and when he bowed he made his back so straight that if water were poured on it, it would have stayed there.’”

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi