Maimuna said that when the Prophet prostrated himself he kept his arms so far away from his sides that if a lamb had wanted to pass under his arms it could have done so.
This is Abu Dawud’s wording, as [Baghawi] explained in Sharh as-sunna with his isnad. Muslim has something to the same effect:
She said that when the Prophet prostrated himself, if a lamb had wanted to pass between his arms it could have done so.