Sunan an-Nasa'i

It was narrated from 'Aishah that she wanted to buy Barirah, but her masters stipulated that her Wala' should go to them. She mentioned that to the Prophet and he said: "Buy her and set her free, for Al-Wala' is to the one who sets the slave free." Some meat was brought and it was said: "This is some of that which was given in charity to Barirah." He said: "It is charity for her and a gift for us." And the Messenger of Allah gave her the choice, and her husband was a free man