From Sahih Muslim (Hadith 1062):
Al-Bara' (b. 'Azib), and he was no liar (but a truthful Companion of the Holy Prophet), reported:They used to say prayer behind the Messenger of Allah (ﷺ). I never saw anyone bending his back at the time when he (the Holy Prophet) raised his head, till the Messenger of Allah (ﷺ) placed his forehead on the ground. They then fell in prostration after him
Reference: Muslim 1062