Ibn Abbas (may Allah be pleased with them both) was asked about a slave who divorced his wife twice, and then both of them were freed: may he marry her again? He said: Yes. It was asked: On what basis? He said: the Messenger of Allah (peace and blessings be upon him) gave a ruling to that effect.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10768