Al-Kahf 18:23
وَلَا تَقُولَنَّ لِشَا۟ىْءٍ إِنِّى فَاعِلٌ ذَٰلِكَ غَدًا
Wala taqoolanna lishay-in innee faAAilunthalika ghada
"And never say of anything, "Indeed, I will do that tomorrow,""
Al-Kahf 18:23, translation by Sahih International
A short reflection
Do not say of anything, I will surely do that tomorrow, except that you add, In sha Allah, if Allah wills.