مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2341
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) left Madinah, heading for Makkah, and he fasted until he reached ‘Usfan. Then he called for a vessel and took it in his hand so that the people could see him, then he broke his fast. And Ibn 'Abbas used to say: Whoever wants to may fast and whoever wants to may not fast.
Musnad Ahmad 2350Its isnad is saheeh, al-Bukhari (4279) and Muslim (1113)]