Musannaf Ibn Abi Shaybah

Yahya bin Saeed reports that it has reached him that Salim bin Abdullah used to say: We have in our possession a written decree from Umar bin Khattab regarding the zakat of camels. No one ever asked us about it until the era of Umar bin Abdul Aziz arrived. So we sent that letter to him, and in that letter it was written that when Umar bin Abdul Aziz sent them to collect zakat, he wrote: "There is no zakat on camels until they reach five."