From Sahih Bukhari (Hadith 811):
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: Bukhari 811