Yahya related to me from Malik from Nafi that Abdullah ibn Umar said, "If someone breaks an oath which he has stressed, he has to free a slave, or clothe ten poor people. If someone breaks an oath, but has not stressed it, he only has to feed ten poor people and each poor person is fed a mudd of wheat. Some one who does not have the means for that, should fast for three days."

Muwatta Malik Book 22, Hadith 13