It is narrated from Sulayman bin Yasar (may Allah have mercy on him) that Sayyiduna Mu'awiyah (may Allah be pleased with him) wrote to Sayyiduna Zayd (may Allah be pleased with him), and Sayyiduna Zayd (may Allah be pleased with him) wrote back: When the divorced woman enters her third menstruation, she has become free of him (the husband).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15386