Mujahid (may Allah have mercy on him) said concerning the words of Allah Almighty, "And those of your women who commit immorality" [Surah al-Nisa: 15]: It means fornication. He said: The command was that they be confined, that is, when four witnesses testify against them: "until death takes them or Allah makes for them a way" [Surah al-Nisa: 15], (the way being) the prescribed punishments (hudud).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / 16906