The Messenger of Allah ﷺ said, "Whoever buys a sheep and finds that its udder has been bound up to make it appear full of milk has a choice. If he wishes, he may return it along with a measure of dates."
ومن ابتاع شاة فوجدها مصراة فهو بالخيار إن شاء ردها وصاعا من تمر
Its isnad is sahih, and it has appeared many times before. See it also in Muslim 3/1334, no. 1710, Abu Dawud 4/196, no. 4593, Tirmidhi 3/25, no. 642, Nasai 5/44-45, no. 2495, and Ibn Majah 2/891, no. 2673.