Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that: "Why does one of you say to his wife: 'I have divorced you, and I have taken you back'? This is not the way Muslims divorce. Divorce women at the beginning of their state of purity."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 14900