Maliks Muwatta Book 22, Hadith Number 10.
Yahya related to me from Malik from Nafi that Abdullah ibn Umar said, “Whoever swears by Allah and then says, ‘Allah willing’ and then does not do what he has sworn to, has not broken his oath.”
Malik said, “The best I have heard on this reservation is that it belongs to the statement made if the speaker does not break the normal flow of speech before he is silent. If he is silent and breaks the flow of speech, he has no exception.”
Yahya said, “Malik said that a man who said that he had disbelieved or associated something with Allah and then he broke his oath, had no kaffara, and he was not a disbeliever or one who associated something with Allah unless his heart concealed something of either of those. He should ask forgiveness of Allah and not return to it – for what he did was evil.”