It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) wrote in a letter regarding the zakat of cows that: "When the number of cows reaches thirty, then a one-year-old male or female cow will be obligatory as zakat, and this ruling will remain until their number reaches forty. When this number reaches forty, then a two-year-old cow will be obligatory, and after that, for every forty, a 'two-year-old cow' will be obligatory."