Sayyiduna Ibn Abbas (may Allah be pleased with them both) held that making an exception (that is, saying "In sha' Allah") is permissible even if it is after a year. Then he recited this verse: "And never say of anything, 'I will do that tomorrow,' except (adding) 'if Allah wills.' And remember your Lord when you forget" [Al-Kahf: 24]. He would say: "That is, when you remember." Al-A'mash was asked: Did you hear this from Mujahid? He said: Layth narrated it to me from Mujahid.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11023