It is narrated from Sayyidah Maymunah (may Allah be pleased with her) that when the Messenger of Allah (peace and blessings be upon him) prostrated, he kept both his arms so far apart from his sides that the whiteness of his armpits could be seen from behind, and when he sat, he rested upon his left thigh.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2706