Musnad Ahmad 27:1954

'Abd al-Samad narrated to us, my father narrated to me, Ayyub narrated to us, from Nafi', from Ibn 'Umar, that the Prophet (peace and blessings of Allah be upon him) said: "Whoever swears an oath and then makes an exception (by saying 'in sha Allah'), then if he wills, he may proceed, and if he wills, he may retract without having broken his oath."