When the Messenger of Allah (ﷺ) prostrated, he kept his hands so much apart from each other that when it was seen from behind the armpits became visible. Waki' said: That is their whiteness.
Sahih Muslim Book 4, Hadith 271
← Previous Hadith 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.

Sahih Muslim Book 4, Hadith 270 Next Hadith → The Messenger of Allah (ﷺ) used to begin prayer with takbir (saying Allahu Akbar) and the recitation: "Praise be to Allah, the Lord of the Universe." When he bowed he neither kept his head up nor bent it down, but kept it between these extremes; when he raised his head after bowing he did not prostrate himself till he had stood erect; when he raised his head after prostration he did not prostrate himself again till he sat up. At the end of every two rak'ahs he recited the tahiyya; and he used to place his left foot flat (on the ground) and raise up the right; he prohibited the devil's way of sitting on the heels, and he forbade people to spread out their arms like a wild beast. And he used to finish the prayer with the taslim.

Sahih Muslim Book 4, Hadith 272