Musnad Ahmad 2:112

Yazid narrated to us, Israel ibn Yunus informed us, from Abd al-A’la al-Tha’labi, from Abd al-Rahman ibn Abi Layla, who said: I was with Umar when a man came to him and said: "I have seen the crescent, the crescent of Shawwal." Umar said: "O people, break your fast." Then he stood up to a vessel containing water, performed ablution, and wiped over his leather socks. The man said: "By Allah, O Commander of the Faithful, I did not come to you except to ask you about this. Have you seen anyone other than you do it?" He said: "Yes, one better than me and better than the [entire] nation. I saw Abu al-Qasim (may Allah bless him and grant him peace) do the same as I have done." He was wearing a Syrian jubbah with tight sleeves, so he inserted his hand from beneath the jubbah. Then Umar prayed the Maghrib prayer.