Sayyiduna Sufyan bin Abdullah (may Allah be pleased with him) narrates that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) sent him as a collector, and he used to count the people's lambs against them. They said: Do you count even our lambs, but take none of them? When he came to Sayyiduna Umar bin al-Khattab (may Allah be pleased with him), he mentioned this, whereupon Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said: Yes, we will count their young, even the one the shepherd carries, but we will not take it; nor will we take the sheep fattened for eating, nor the one with young, nor the pregnant one, nor the stud of the flock; rather we will take sheep of jadha'ah or thaniyyah age, and that is fairness between the young of the wealth and its best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7302