When the Messenger of Allah (ﷺ) prostrated himself, if a lamb wanted to pass between his arms, it could pass.
Sahih Muslim Book 4, Hadith 269
← Previous Hadith " When the Messenger of Allah (rtiay peace be upon him) prostrated, he spread out his arms so that the whiteness of his armpits was visible." And in the narration transmitted by al-Laith (the words are:" When the Messenger of Allah (ﷺ) prostrated. he spread his hands from the armpits so that I saw their whiteness."

Sahih Muslim Book 4, Hadith 268 Next 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