It is narrated from Sayyiduna Amr bin Hazm (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) had a written document prepared for him, in which it was written: "The zakat of cows is that for every thirty cows, a one-year-old calf is due, and for every forty cows, a two-year-old calf is due as zakat."