Musannaf Ibn Abi Shayba 9:121

Hushaym narrated to us, from Mughira, from Ibrahim, who said: I asked him and said, "Did they use to stipulate at the time of the marriage contract [the condition of] 'retaining in kindness or releasing with benevolence'?" He said: He replied, "That is [incumbent] upon them even if they do not stipulate it. Our companions did not use to stipulate it."