It is narrated from Sa'd (may Allah be pleased with him), he states that two men came to me and said: "The Messenger of Allah (ﷺ) has sent us so that we may collect zakat from the people." Sa'd (may Allah be pleased with him) said: "I brought out for them a pregnant goat, which was the best among my goats, but they refused to take it and said: The Messenger of Allah (ﷺ) has forbidden us from taking a pregnant goat (as zakat)." Sa'd (may Allah be pleased with him) says: "I then gave them a goat of average quality, and they accepted it."
Hadith Referenceمسند الإمام الشافعي / كتاب الزكاة / 699
Hadith Takhrijاخرجه ابوداود، الزكاة، باب في زكاة السائمة (1581) والنسائى الزكاة، باب اعطاء السيد المال بغير اختيار المصدق (2464)، (2465)۔