The Quran

وَاللّاتي يَأتينَ الفاحِشَةَ مِن نِسائِكُم فَاستَشهِدوا عَلَيهِنَّ أَربَعَةً مِنكُم ۖ فَإِن شَهِدوا فَأَمسِكوهُنَّ فِي البُيوتِ حَتّىٰ يَتَوَفّاهُنَّ المَوتُ أَو يَجعَلَ اللَّهُ لَهُنَّ سَبيلًا

(Saheeh International)

Those who commit immorality [i.e., unlawful sexual intercourse] of your women – bring against them four [witnesses] from among you. And if they testify,¹ confine them [i.e., the guilty women] to houses until death takes them or Allāh ordains for them [another] way.²

The witnesses must swear to actually having seen the act taking place.
The "other way" (i.e., penalty) was later revealed in 24:2, canceling the ruling in this verse.