مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2735
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas (رضي الله عنه) said: I saw the Messenger of Allah (ﷺ) prostrating with his arms held away from his body, and I saw the whiteness of his armpits.
Musnad Ahmad 2753Saheeh because of corroborating evidence]