Sahih Muslim
The Book of Divorce (Kitab Al-Talaq)
Book Number: 009 / Hadith Number: 3483
Ayyub reported a hadith like this with the same chain of narrators and he said: Umar (Allah be pleased with him) asked Allah's Apostle (may peace be upon him) about it and he commanded him that he should take her back until she is divorced in the state of purity without having a sexual intercourse with her, and said: Divorce her in the beginning of her 'Idda or her 'Idda commences.