مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2719
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas (رضي الله عنه) said: By Allah, the Messenger of Allah (ﷺ) did not fast any month in full except Ramadan, When he fasted, he would fast until one would think, By Allah, he will never stop fasting. And he would refrain from fasting until one would think, By Allah, he will never fast.
Musnad Ahmad 2737Its isnad is saheeh, al-Bukhari (1971) and Muslim (1157)]