The Albaani Site

The Imaam said, “The brother here ­also reminded [us of] another ­­calamity common amongst those who pray and which is in opposition to the Sunnah: Al-Baraa ibn ’Aazib, may Allaah be pleased with him, said that when we would pray behind the Prophet ﷺ we would not prostrate until we saw that the Prophet ﷺ had placed his forehead on the ground.

Nowadays after saying, ‘Sami’Allaahu liman hamidah, Rabbanaa wa lakal-Hamd,’ the Imaam will barely have said, ‘Allaahu Akbar,’ except that the people [behind him] will have fallen down into prostration with him—this is a mistake.

He would say, ‘Allaahu Akbar,’ and when he was seen to have finished going into prostration and had placed his forehead on the ground, it was then that the row behind him would start to go into prostration with him ﷺ.”

Al-Hudaa wan-Noor, 183.