When we were (in prayer) with the Messenger of Allah Allah (ﷺ) none of us benfft his back till we saw he prostrated. Zuhair and others reported:" till we saw him prostrating".
Sahih Muslim Book 4, Hadith 227
← Previous Hadith They (the Companions) said prayer with the Messenger of Allah (ﷺ), and they bowed when he (the Holy Prophet) bowed. and when he raised his head after bowing, he pronounced:" Allah listened to him who praised Him," and we kept standing till we saw him placing his face on the ground and then we followed him.

Sahih Muslim Book 4, Hadith 226 Next Hadith → I said the dawn prayer behind the Apostle of (ﷺ) and heard him reciting: 'Nay. I call to witness the stars, running their courses and setting" (al-Qur'an, lxxxi. 15-16) and Done of us bent his back till he completed prostration.

Sahih Muslim Book 4, Hadith 228