Musannaf Ibn Abi Shayba 7:416

'Abd al-Samad ibn 'Abd al-Warith narrated to us, from Hammad ibn Salamah, from 'Abd Allah ibn 'Utbah, that a woman vowed to perform i'tikaf for ten days, but she died and did not perform the i'tikaf. So Ibn 'Abbas said to her son, "Perform the i'tikaf on behalf of your mother."