Yahya related to me from Malik from Hisham ibn Urwa from hisfather that A'isha, umm al-muminin said, "Rashness in oaths is that aman says, 'By Allah, No! by Allah!' " i.e. out of habit.Malik said, "The best of what I have heard on the matter is thatrashness in oaths is that a man take an oath on something to show thathe is certain that it is like he said, only to find that it is otherthan what he said. This is rashness."Malik said, "Thebinding oath is for example, that a man says that he will not sell hisgarment for ten dinars, and then he sells it for that, or that he willbeat his young slave and then does not beat him, and so on. One doeskaffara for making such an oath, and there is no kaffara in rashness."Malik said, "As for the one who swears to a thing which heknows is wicked, and he swears to a lie he knows to be a lie, in orderto please someone with it or to excuse himself to someone by it or togain money by it, no kaffara that he does for it can cover it."
USC-MSA web (English) reference: Book 22, Hadith 9
Arabic reference: Book 22, Hadith 1021