When the Messenger of Allah (ﷺ) prostrated himself, he kept his arms far away from his sides so much so that we took pity on him.
Sunan Abi Dawud Book 2, Hadith 510
← Previous Hadith I came to the Prophet (ﷺ) from behind. I saw the whiteness of his armpits and he kept his arms away from his sides and raised his stomach (from the ground).

Sunan Abi Dawud Book 2, Hadith 509 Next Hadith → when one of you prostrates himself, he should not stretch out his forearms( on the ground) like a dog and he should join both of his thighs.

Sunan Abi Dawud Book 2, Hadith 511