From Sahih Muslim (Hadith 1108):
Maimuna, the wife of the Messenger of Allah (ﷺ), reported:When the Messenger of Allah (ﷺ) prostrated himself, he spread his arms, i. e. he separated them so much that the whiteness of his armpits became visible from behind and when he sat (for Jalsa) he rested on his left thigh
Reference: Muslim 1108