Musnad Ahmad 27:644

Isma'il narrated to us, Ayyub narrated to us, from Nafi', from Ibn 'Umar, who said: I do not know it except from the Prophet, may Allah bless him and grant him peace, who said: "Whoever swears an oath and then makes an exception (by saying 'in sha Allah'), he has the choice: if he wishes, he may proceed with his oath, and if he wishes, he may retract without having broken it," or he said: "without sin."