Hasan (may Allah have mercy on him) said: "Whoever pronounces zihar on his wife, then divorces her, then she marries someone else, then he divorces her, and the first husband marries her again, it is obligatory for him to offer expiation before he can go to his wife."
Hadith Referenceسنن سعید بن منصور / كتاب الطلاق / 2997