Musnad Ahmad 27:1627

Yunus narrated to us, Hammad—meaning Ibn Salamah—narrated to us, from Ayyub, from Nafi’, from Ibn Umar, that the Messenger of Allah (ﷺ) said: "If one of you swears an oath and says 'If Allah wills,' then he has the choice: if he wills, he may act, and if he wills, he may not act."