مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3282
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Shu'bah said: Ibn ‘Abbas (رضي الله عنه) saw a man prostrating with his forearms on the ground. Ibn 'Abbas said: This is how the dog sits. I saw The Messenger of Allah (ﷺ) when he prostrated, I could see the whiteness of his armpits.
Musnad Ahmad 3305Saheeh; because of corroborating evidence; this is a da’eef