Sayyiduna Ibn Abbas (may Allah be pleased with them both), regarding the saying of Allah, the Mighty and Majestic: "And remember your Lord when you forget" [Al-Kahf: 24], said: "That is, when you forget to say 'In sha' Allah' (if Allah wills), then say it when you remember. This concession was specifically for the Messenger of Allah (peace and blessings be upon him); it is not permissible for us to make an exception except when it is joined to the oath."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11097