مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2924
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas said: By Allah, the Messenger of Allah (ﷺ) never fasted any month in full except Ramadan. When he fasted, he would fast until one would say: By Allah, he will never stop fasting. And he would refrain from fasting until one would say: By Allah, he would never fast.
Musnad Ahmad 2947lts isnad is saheeh]