Silsilat al-Ahadith al-Sahihah

Sayyiduna Bara’ bin Azib (may Allah be pleased with him) says that the Companions used to pray with the Messenger of Allah (peace and blessings be upon him). When he would bow (ruku‘), they would bow; when he would say «سمع الله لمن حمده», the Companions would remain standing (after rising from ruku‘), and when they saw that he had placed his face or forehead on the ground (for prostration), then following him, they would also go into prostration.