(133) باب السُّجُودِ عَلَى سَبْعَةِ أَعْظُمٍ
Narrated Al-Bara' bin `Azib:
(He was not a liar) We used to pray behind the Prophet (ﷺ) and when he said, "Sami`a l-lahu liman hamidah", none of us would bend his back (to go for prostration) till the Prophet (ﷺ) had placed his, forehead on the ground.
Reference: Sahih al-Bukhari 811
In-book reference: Book 10, Hadith 206
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 12, Hadith 775