Al-Qasim bin Muhammad (may Allah have mercy on him) said: He would divide them into three parts: one third the best, one third middling animals, and one third weaker than that; the collector would leave the best and collect from the middling. It is reported from Umar bin al-Khattab (may Allah be pleased with him) that the owner of the sheep would choose one third, and then they would choose from the remaining two thirds.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7313