Musannaf Ibn Abi Shaybah

It is narrated from Hisham that Muhammad said: People used to make wills in such a way that a man would write in his will, "If death comes to me before I change my will." If he feels the need to make a change, he can do so, whether it is a will for the emancipation of a slave or something else. And if he had not set any condition in the will, even then he can change the will, except in the case of the emancipation of a slave.