Musnad Abu Ya'la al-Mawsili 159:40

Bishr ibn al-Walid narrated to us, Abd al-Rahman ibn Abi al-Zinad narrated to us, from his father, from al-A’raj, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The best charity is a milch camel that is productive, given as a gift, or a milch sheep that is productive; it goes out in the morning with one vessel and returns in the evening with another."