مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3837
Musnad of Abdullah bin Mas'ood
It was narrated that `Abdullah (رضي الله عنه) said: The Messenger of Allah (ﷺ) used to fast three days at the beginning of every month, and he rarely did not fast on a Friday.
Musnad Ahmad 3860Its isnad is hasan]