Musannaf Ibn Abi Shayba 25:89

Yahya ibn Sa'id narrated to us, from Husayn al-Mu'allim, from 'Amr ibn Shu'ayb, from 'Abdullah ibn al-Harith ibn Abi Rabi'ah—or al-Harith ibn 'Abdullah ibn Abi Rabi'ah—who said: I said to 'Umar: "There is a thing that the people of Yemen do: a man makes a will, then he changes his will." He said: "Let him change whatever he wishes of his will."