Al-Bara' reported: When we were (in prayer) with the Messenger of Allah Allah (صَلَّی اللَّهُ عَلَيْهِ وَسَلَّمَ) none of us benfft his back till we saw he prostrated. Zuhair and others reported:" till we saw him prostrating".
Hadith ReferenceSahih Muslim / The Book of Prayers / Hadith: 474