Sayyiduna Anas (may Allah be pleased with him) said that Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) wrote for him: This is the obligation of zakah which the Messenger of Allah (peace and blessings be upon him) prescribed for the Muslims. Then he narrated the whole hadith, in which there is also: "Those grazing separately are not to be combined, and those grazing together are not to be separated, for fear of zakah; and those who are partners will settle between themselves equally."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7329