Musnad Ahmad 2:226

Yazid narrated to us, Waraqa informed us. And Abu al-Nadr said: Waraqa narrated to us, from Abd al-A’la al-Tha’labi, from Abd al-Rahman ibn Abi Layla, who said: I was with al-Bara’ ibn Azib and Umar ibn al-Khattab in al-Baqi’, looking at the crescent moon. A rider approached, and Umar met him and said: "From where have you come?" He said: "From the west." He said: "Did you sight the crescent?" He said: "Yes." Umar said: "Allahu Akbar (God is the Greatest). It is sufficient for the Muslims [that there be] one man." Then Umar stood up and performed ablution, and he wiped over his leather socks, then he prayed the Maghrib prayer. Then he said: "This is how I saw the Messenger of Allah, peace and blessings of Allah be upon him, do." Abu al-Nadr said: And he was wearing a cloak with tight sleeves, so he brought his hand out from underneath it and wiped.