مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3422
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibraheem said: I heard that when the Prophet (ﷺ) prostrated, the whiteness of his armpits could be seen.
Musnad Ahmad 3446lts isnad is da'eef, because it is mursal]