Muwatta Imam Malik Book of Divorce, Hadith 69

Malik said, "What is done among us when a slave divorces a slave- girl when she is a slave and then she is set free, is that her idda is the idda of a slave-girl, and her being set free does not change her idda whether or not he can still return to her. Her idda is not altered."
Malik added, "The hadd-punishment which a slave incurs is the same as this. When he is freed after he has incurred but before the punishment has been executed, his hadd is the hadd of the slave."
Malik said, "When a free man divorces a slave-girl three times, her idda is two periods. When a slave divorces a free woman twice, her idda is three periods."
Malik said about a man who had a slave-girl as a wife, and he bought her and set her free, ''Her idda is the idda of a slave-girl, i.e. two periods, as long as he has not had intercourse with her. If he has had intercourse with her after buying her and before he set her free, she only has to wait until one period has passed . "