Jabir (may Allah have mercy on him) narrates from al-Sha'bi (may Allah have mercy on him) concerning a man who said to his wife: "When Ramadan begins, you are divorced." He said: "She remains his wife from the day he pronounced the divorce until Ramadan begins."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 15092