Sunan An Nasai 1106

It was narrated from Abdullah bin Malik bin Buhainah that: When the Messenger of Allah (ﷺ) prayed he held his arms out so much that the whiteness of his armpits appeared. ..

It was narrated that Abu Hurairah said: If I were in front of the Messenger of Allah (ﷺ) I would be able to see the whiteness of his armpits. (One of the narrators) Abu Mijlaz said: It is as if he said that because he was praying. ..

It was narrated from 'Ubaidullah bin Abdullah bin Aqram that his father said: I prayed with the Messenger of Allah (ﷺ) and I used to see the whiteness of his armpits when he prostrated. ..

It was narrated from Maimunah that: When the Prophet (ﷺ) prostrated he would hold his arms out from his sides so that if a lamb wanted to pass beneath his arms it would be able to do so. ..

It was narrated that Qatadah said: I heard Anas (narrate) that the Messenger of Allah (ﷺ) said: 'Be moderate in prostration and do not rest your forearms along the ground like a dog.' ..

It was narrated that Abu Mas'ud said: The Messenger of Allah (ﷺ) said: 'No prayer is valid in which a man does not maintain his back (at ease) when bowing and prostrating.' ..