Quran 70:31 — The Heights (Al-Ma'aarej)

فَمَنِ ابتَغىٰ وَراءَ ذٰلِكَ فَأُولٰئِكَ هُمُ العادونَ

anyone who transgresses these limits is a sinner.

[70:30] (They have relations) only with their spouses, or what is legally theirs -

[70:32] And the believers keep their word; they are trustworthy.