Muhammad bin Amr bin Ata (may Allah have mercy on him) said: I heard Abu Humayd al-Sa'idi (may Allah be pleased with him) speaking among a group of ten of the Companions of the Messenger of Allah (peace and blessings be upon him), among whom was Abu Qatadah (may Allah be pleased with him). Then he mentioned the complete hadith about the manner of the prayer of the Messenger of Allah (peace and blessings be upon him); in it, it is also mentioned that he would go down to the ground in prostration and keep his hands (arms) away from his sides, then he would raise his head, fold his left leg and sit on it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2730