Imam al-Zuhri narrates from Salim (may Allah have mercy on him), and he from his father (Sayyiduna Abdullah bin Umar, may Allah be pleased with them both), that the Messenger of Allah (peace and blessings be upon him) wrote down the scale of zakah but had not yet sent it out to his governors when he (peace and blessings be upon him) passed away, so the document was tied to his (peace and blessings be upon him) sword. Sayyiduna Abu Bakr (may Allah be pleased with him) implemented it until he passed away, then Sayyiduna Umar (may Allah be pleased with him) acted upon it until he too passed away. He narrated the hadith on the zakah of camels and also of sheep, and said: "Those grazing together are not to be separated, and those grazing separately are not to be combined, for fear of zakah; and those who are partners will settle equally between themselves."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7330