Musannaf Ibn Abi Shayba 7:415

Jarir narrated to us, from Layth, who said: Tawus was asked about a woman who died while she had a vow to perform i'tikaf for a year in the Sacred Mosque, and she had four sons, all of whom wanted to fulfill it on her behalf. Tawus said: "All four of you perform i'tikaf in the Sacred Mosque for three months, and fast."