Sunan Ibn Majah, Chapter: The Chapters on Legal Punishments

that Umar bin Khattab said: "I fear that after a long time has passed, some will say: 'I do not find (the sentence of) stoning in the Book of Allah (saw),' and they will go astray by abandoning one of the obligations enjoined by Allah (SWT). Rather stoning is a must if a man is married (or previously married) and proof is established, or if pregnancy results or if he admits it. I have read it (in the Quran). "And if an old man and an old woman commit adultery, stone them both." The Messenger of Allah (saw) stoned (adulterers) and we stoned (them) after him.' " Sahih