Musannaf Ibn Abi Shaybah

Ibrahim says that if a person makes a will in favor of an heir and before his death his heirs give their consent, then after his death if they change their decision, they have the right to do so. And if a will is made for a non-heir for more than one-third of the wealth, the same rule applies. And if a will is made for a non-heir for less than one-third, then it becomes effective.