Imam Malik's Muwatta, Chapter: Zakat

Yahya related to me from Malik from Yahya ibn Said from Muhammad ibn Yahya ibn Habban from al-Qasim ibn Muhammad that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, said, "Sheep from the zakat were brought past Umar ibn al-Khattab and he saw amongst them a sheep with a large udder, ready to give milk, and he said, 'What is this sheep doing here?' and they replied, 'It is one of the sheep from the zakat.' Umar said, 'The owners did not give this sheep willingly. Do not subject people to trials. Do not take from the muslims those of their animals which are the best food-producers.' "