Musnad Ahmad 27:63

Isma'il narrated to us, Ayyub narrated to us, from Nafi', from Ibn 'Umar—Ayyub said: I do not know it except—from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever swears an oath, then makes an exception (by saying 'in sha Allah'), he has the choice. If he wishes to proceed with his oath, he proceeds; and if he wishes to retract, he does so without breaking the oath," or he said, "without sin."