مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3453
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
’Ata' said: 'Abdullah bin 'Abbas called al-Fadl bin 'Abbas to eat on the day of Arafah, and he said: I am fasting. ‘Abdullah said: Do not fast, for a vessel of milk was brought to the Prophet (ﷺ) on the day of 'Arafah and he drank from it on this day. So do not fast, for people follow your example.
Musnad Ahmad 3476A saheeh hadeeth; there is an interruption in this isnad bet