Oath-taking (Qasm) in the Quran

فَوَرَبِّ ٱلسَّمَآءِ وَٱلْأَرْضِ إِنَّهُۥ لَحَقٌّ مِّثْلَ مَآ أَنَّكُمْ تَنطِقُونَ

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