Asbat narrates from al-Suddi regarding the verse of ila that Sayyiduna Ali and Ibn Abbas (may Allah be pleased with them both) used to say: When a man takes an oath of ila from his wife, then after four months have passed he will be made to stand, and it will be said to him: "Will you keep your wife or divorce her?" If he keeps her, she is his wife; if he divorces her, it is an irrevocable divorce. Sayyiduna Abdullah bin Mas'ud and Umar bin al-Khattab (may Allah be pleased with them both) used to say: When four months pass, this woman is irrevocably divorced, and she has more right over herself.
Imam al-Shafi'i (may Allah have mercy on him) said, regarding their using the statement of Ibn Abbas (may Allah be pleased with them both) as evidence: We said: As for Ibn Abbas, you yourself differ from him on ila. He said: How so?
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإيلاء / 15232