Musnad Ahmad 27:1109

Yazid narrated to us, Sa'id ibn Abi 'Arubah informed us, from Ma'mar, from al-Zuhri, from Salim, from Ibn 'Umar, who said: Ghilan ibn Salamah al-Thaqafi accepted Islam while he had ten wives from the Jahiliyyah (pre-Islamic period), and they accepted Islam with him. The Prophet, peace and blessings of Allah be upon him, ordered him to choose four of them.