Surah Al-Kahf 18:23

وَلَا تَقُولَنَّ لِشَا۟ىْءٍ إِنِّى فَاعِلٌ ذَٰلِكَ غَدًا

And never say of anything, "Indeed, I will do that tomorrow,"

Translation: Saheeh International

Tafsir

And never say regarding something that is for the purpose of doing something ‘I will indeed do that tomorrow’ in other words I will do something at some future point in time

Excerpt from Tafsir al-Jalalayn, commentary on 18:23, via Quran.com. The reader has the full commentary and 9 more tafsirs.