مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2393
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas (رضي الله عنه) said: I came to the Messenger of Allah (ﷺ) from behind and I saw the whiteness of his armpits when he was prostrating with his arms held away from his body.
Musnad Ahmad 2405Saheeh because of corroborating evidence]