"Ibn 'Abbas was asked about a slave who divorces his wife twice, then (they are freed). Can he marry her? He said: 'Yes.' It was said to him: 'On what basis?' He said: 'The Messenger of Allah (ﷺ) passed such a judgement .' " (D a' if)
Sunan Ibn Majah Book 10, Hadith 67
← Previous Hadith "A man came to the Prophet (ﷺ) and said: 'O Messenger of Allah, my master married me to his slave woman, and now he wants to separate me and her.' The Messenger of Allah (ﷺ) ascended the pulpit and said: 'O people, what is the matter with one of you who marries his slave to his slave woman, then wants to separate them? Divorce belongs to the one who takes hold of the calf (i.e., her husband).’ “

Sunan Ibn Majah Book 10, Hadith 66 Next Hadith → "Do not corrupt the Sunnah of our Prophet Muhammad (ﷺ). The waiting period of an Umm Walad is four months and ten (days)."

Sunan Ibn Majah Book 10, Hadith 68