Musnad Ahmad 1:49

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: I heard Abu Ishaq saying: I heard al-Bara' saying: When the Messenger of Allah, may Allah bless him and grant him peace, came from Makkah to al-Madinah, the Messenger of Allah, may Allah bless him and grant him peace, became thirsty. They passed by a shepherd of sheep. Abu Bakr al-Siddiq said: So I took a cup and milked into it for the Messenger of Allah, may Allah bless him and grant him peace, a small amount of milk. I brought it to him, and he drank until I was satisfied.