It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever swears an oath about something and also says 'In sha Allah' (if Allah wills), then he has the choice: if he wishes, he may fulfill the oath, and if he wishes, he may not fulfill it (i.e., he may break the oath)."