Abu Dawud Book 6, Hadith Number 2182.

Narated By Abdullah ibn Abbas : Umar ibn Mu’tab reported: AbuHasan, a client of Banu Nawfal asked Ibn Abbas: A slave had a wife who was a slave-girl. He divorced her by two pronouncements. Afterwards both of them were freed. Is it permissible for him to ask her in marriage again? He said: Yes. This is a decision given by the Apostle of Allah (PBUH).