It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with him): When the Messenger of Allah (peace and blessings be upon him) prostrated, he would keep his arms so far apart from his sides that the whiteness of his (peace and blessings be upon him) armpits could be seen.
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1732