It is narrated from Sayyiduna Adi bin Amirah al-Kindi (may Allah be pleased with him) that when the Messenger of Allah (peace and blessings be upon him) prostrated, he kept 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 العين / 14335