Musannaf Ibn Abi Shayba 9:815

Ghundar narrated to us, from Shu'bah, from Mansur, who said: Mujahid narrated to us in the room of al-Minhal ibn 'Amr, from Ibn Abi Layla, from Umar ibn al-Khattab, that he said regarding the wife of a missing person: She shall wait for four years, then his guardian shall be summoned, and he shall divorce her. Then she shall observe an 'iddah (waiting period) after that of four months and ten [days].