Musannaf Ibn Abi Shayba 7:453

12597. Ibn ‘Ulayyah narrated to us, from ‘Ali ibn al-Hakam al-Bunani, from Maymun, from Ibn ‘Abbas (may Allah be pleased with them both), that he was asked about a man who died while he had a vow (nadhr) upon him. He said: "The vow is to be fasted on his behalf."