مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2437
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn ‘Abbas (رضي الله عنه)said: The Messenger of Allah (ﷺ) never fasted any month in full except Ramadan, even though he used to fast until one would think: By Allah, he will never stop fasting; and he used not to fast until one would think: By Allah, he will never fast.
Musnad Ahmad 2450Its isnad is saheeh, al Bukhari (1971) and Muslim (1157)]