Sayyiduna Anas bin Malik (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever buys a goat whose milk has been held back: after milking the animal, if he likes it he may keep it; otherwise he should return it and also give a sa' of dates."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10725