مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2147
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) used to fast until we thought that he did not want to stop fasting, and he would not fast until we thought that he did not want to fast, but he did not fast any month in totally apart from Ramadan from the time he came to Madinah.
Musnad Ahmad 2151Its isnad is saheeh, Muslim (1157)]