Abbas bin Sahl narrated that Abu Humayd, Abu Usayd, Sahl bin Sa'd and Muhammad bin Maslamah (may Allah be pleased with them) were gathered (in one place), and they mentioned the prayer of the Prophet (peace and blessings be upon him). Abu Humayd (may Allah be pleased with him) said: "I know the prayer of the Prophet (peace and blessings be upon him) better than any of you." Then he mentioned the full hadith, and then said that he (peace and blessings be upon him) bowed and placed his hands on his knees as though he were gripping them, made his arms taut like a bowstring, and kept them away from his sides.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2551