Musnad Ahmad 27:1643

'Affan narrated to us, Wuhaib narrated to us, Ayyub narrated to us, from Nafi', from Ibn 'Umar, from the Prophet (peace and blessings of Allah be upon him), who said: "If a man swears an oath and says 'If Allah wills,' then he has the choice: if he wishes, he may proceed, and if he wishes, he may leave it."