Musannaf Ibn Abi Shaybah

Ibrahim says that when someone is told, "You have sworn that you will not do such and such?" and he replies, "Alright," but does not actually swear, it is said that expiation for an oath is due upon him.

Hasan says that when a person says, "I swear," and then breaks it, then expiation is due upon him.

Hazrat Hammad says that when a person says, "I have taken an oath," even though he has not actually taken an oath, then there is no expiation (kaffarah) upon him.

Ibrahim says that when a person says, "I have taken an oath," while in fact he has not sworn an oath, then indeed he has lied and has become a swearer, and if he says, "Indeed, I have taken an oath and I lied," then indeed he has lied.