Yahya related to me from Malik from Yahya ibn Said from Muhammadibn Yahya ibn Habban from al-Qasim ibn Muhammad that A'isha, the wifeof the Prophet, may Allah bless him and grant him peace, said, "Sheepfrom the zakat were brought past Umar ibn al-Khattab and he sawamongst them a sheep with a large udder, ready to give milk, and hesaid, 'What is this sheep doing here?' and they replied, 'It is one ofthe sheep from the zakat.' Umar said, 'The owners did not give thissheep willingly. Do not subject people to trials. Do not take from themuslims those of their animals which are the best food-producers.' "
USC-MSA web (English) reference: Book 17, Hadith 28
Arabic reference: Book 17, Hadith 605