Sayyiduna Ahmar bin Jaz' (may Allah be pleased with him), who was a Companion of the Messenger of Allah (peace and blessings be upon him), said: "We used to feel pity for the Messenger of Allah (peace and blessings be upon him) because he (peace and blessings be upon him) kept his arms so far away from his sides when in prostration."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 809