Sayyiduna Anas (may Allah be pleased with him) said that Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) sent him a written document, and in it he wrote: This is the obligation (the prescribed scale) which the Messenger of Allah (peace and blessings be upon him) laid down for the Muslims, which Allah and His Messenger have commanded. Then he narrated the hadith, and in it was: "There is zakah on grazing sheep."
Sayyiduna Umar (may Allah be pleased with him) said: "When grazing sheep number from forty up to one hundred and twenty, there is one sheep due on them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7388