مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2923
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
‘Ata’ narrated that 'Abdullah bin ‘Abbas (رضي الله عنه) invited al-Fadl to eat on the day of 'Arafah, and he said: I am fasting. 'Abdullah said: Do not fast, for some milk was brought to the Prophet (ﷺ) and he drank it on this day; and, moreover, people follow your example.
Musnad Ahmad 2946A saheeh hadeeth]