Adh-Dhariyat 51:23

Fawarabbi assama-i wal-ardiinnahu lahaqqun mithla ma annakum tantiqoon

Then by the Lord of the heaven and earth, indeed, it is truth - just as [sure as] it is that you are speaking.

So, by the Lord of the heavens and the earth, it (the Day of Recompense) is a reality, as sure as the fact that you (can) speak.