مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3138
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated from Ibn ‘Abbas (رضي الله عنه) that The Messenger of Allah (ﷺ) set out from Madinah in Ramadan when he conquered Makkah, and he fasted until he reached 'Usfan, then he called for a vessel and drank, Ibn ‘Abbas (رضي الله عنه) used to say: Whoever wishes may fast and whoever wishes may not fast.
Musnad Ahmad 3162Its isnad is saheeh, al-Bukhari (4279) and Muslim (1113)]