Musnad Ahmad Hadith 25786

It is narrated from Aisha (may Allah be pleased with her) that once Barirah came to her; she was a mukatabah (a slave woman who had a contract for her freedom) and came with a request for help in paying her contract amount. Aisha (may Allah be pleased with her) asked her, "Do your owners want to sell you? If they wish, I will pay your contract amount, but your wala (right of inheritance/loyalty) will be mine." She went to her owners and mentioned this to them. They said, "Not unless she accepts the condition that your inheritance will go to us." The Prophet (ﷺ) said to Aisha (may Allah be pleased with her), "Buy her and set her free, for the wala (right of inheritance/loyalty) belongs only to the one who sets the slave free." Then the Prophet (ﷺ) stood up and said, "What is wrong with people that they stipulate conditions which are not in the Book of Allah? Whoever stipulates a condition that is not permitted in the Book of Allah, it is not valid, even if he stipulates it a hundred times. The condition of Allah is more deserving and stronger."