Sayyidah Umm al-Fadl (may Allah be pleased with her) said: The people were in doubt about whether the Prophet (peace and blessings be upon him) was fasting on the day of Arafah, so I said: "I will find this out for you." So I sent him (peace and blessings be upon him) a vessel full of milk, and he (peace and blessings be upon him) drank it.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21500