Muwatta Malik, Book 29, Hadith 1238 (In-Book)

Yahya related to me from Malik that Ibn Shihab said, "I have heard that the Messenger of Allah, may Allah bless him and grant him peace, said to a man from Thaqif who had ten wives when he became muslim, 'Take four and separate from the rest.' "

وَحَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، أَنَّهُ قَالَ بَلَغَنِي أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ لِرَجُلٍ مِنْ ثَقِيفٍ أَسْلَمَ وَعِنْدَهُ عَشْرُ نِسْوَةٍ حِينَ أَسْلَمَ الثَّقَفِيُّ ‏ "‏ أَمْسِكْ مِنْهُنَّ أَرْبَعًا وَفَارِقْ سَائِرَهُنَّ ‏" ‏ ‏.‏

sunnah.com

also referenced under: Book 29, Hadith 76 (USC-MSA)