It is narrated from Nafi' that Ibn Umar (may Allah be pleased with him) was asked about the 'iddah of an umm walad, and he said: Her 'iddah is one menstrual period. A man said that Uthman (may Allah be pleased with him) says: Her 'iddah is three menstrual periods. Ibn Umar (may Allah be pleased with him) said: Uthman (may Allah be pleased with him) is the best of us and the most knowledgeable of us. There is weakness in its chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15585