Muhammad bin Abi Suwayd (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) said to Ghaylan bin Salamah al-Thaqafi (may Allah be pleased with him), at the time he accepted Islam while he had ten wives: "Choose four of them and separate from the rest."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14046