It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) concerning the saying of Allah Almighty: "And never say of anything, 'I will do that tomorrow,' except (with the words) 'If Allah wills,' and remember your Lord when you forget" [Surah al-Kahf: 24]. He said: "That is, that you say: In sha Allah (if Allah wills)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12762