Musannaf Ibn Abi Shayba 7:176

Hafs narrated to us, from Layth, from Hammad, from Ibrahim, who said: Oaths are four. Two of them require expiation: "By Allah, I will do [it]" and "By Allah, I will do [it]." He said: So these two require expiation. And "By Allah, I did not do [it]" and "By Allah, I will do [it]" do not require expiation.