Sayyidah Aishah (may Allah be pleased with her) said: Sheep of charity were passed by Umar bin al-Khattab (may Allah be pleased with him), and he saw among them a milk-laden sheep with a huge udder. Umar (may Allah be pleased with him) asked: "What sheep is this?" They said: "It is a sheep of zakah." Umar (may Allah be pleased with him) said: "Its owners would not have given this willingly. So do not put the people to trial. Do not take the choicest wealth of the Muslims; keep away from the food (the milk-giving animals)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7660