Sahih Muslim Book 17, Hadith Number 4194.

‘Abdullah b. ‘Abbas reported that ‘Umar b. Khattab sat on the pulpit of Allah’s Messenger (may peace be upon him) and said: Verily Allah sent Muhammad (may peace be upon him) with truth and He sent down the Book upon him, and the verse of stoning was included in what was sent down to him. We recited it, retained it in our memory and understood it. Allah’s Messenger (may peace be upon him) awarded the punishment of stoning to death (to the married adulterer and adulteress) and, after him, we also awarded the punishment of stoning, I am afraid that with the lapse of time, the people (may forget it) and may say: We do not find the punishment of stoning in the Book of Allah, and thus go astray by abandoning this duty prescribed by Allah. Stoning is a duty laid down in Allah’s Book for married men and women who commit adultery when proof is established, or it there is pregnancy, or a confession.