It is narrated from Sayyidah Maymunah (may Allah be pleased with her) that the people were in doubt about the Prophet's (peace and blessings be upon him) fasting on the day of Arafah, so Maymunah (may Allah be pleased with her) sent him (peace and blessings be upon him) a vessel of milk while he (peace and blessings be upon him) was standing at Arafat, and he (peace and blessings be upon him) drank from it while the people were watching.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20430