مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2761
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas (رضي الله عنه) said: I prayed behind the Messenger of Allah (ﷺ) and I saw him holding his arms away from his body, and I saw the whiteness of his armpits.
Musnad Ahmad 2781Saheeh because of corroborating evidence]