Musnad Abu Ya'la al-Mawsili 1:111

Ubayd Allah ibn Mu'adh al-'Anbari narrated to us, my father narrated to us, Shu'bah narrated to us, from Abu Ishaq, from al-Bara' ibn 'Azib, who said: Abu Bakr al-Siddiq said: When we departed with the Messenger of Allah (may Allah bless him and grant him peace) from Makkah to Madinah, we passed by a shepherd. The Messenger of Allah (may Allah bless him and grant him peace) became thirsty. So I milked a small amount of milk for him, and I brought it to him, and he drank until I was satisfied.