Sunan Ibn Majah, Chapter: The Chapters on Divorce

'Three Sunan were established because of Barirah: She was given the choice (of whether to remain married) when she was freed, and her husband was a slave; they used to give her charity and she used to give it as a gift to the Prophet (saw), and he would say: 'It is charity for her and a gift for us,' and he said, the 'Wala' is for the one who set the slave free.'"