Maliks Muwatta Book 41, Hadith Number 10.
Malik related to me that Yahya ibn Said heard Said ibn al-Musayyab say, “When Umar ibn al-Khattab came from Mina, he made his camel kneel at al-Abtah, and then he gathered a pile of small stones and cast his cloak over them and dropped to the ground. Then he raised his hands to the sky and said, ‘O Allah! I have become old and my strength has weakened. My flock is scattered. Take me to You with nothing missed out and without having neglected anything.’ Then he went to Madina and addressed the people. He said, ‘People! Sunan have been laid down for you. Obligations have been placed upon you. You have been left with a clear way unless you lead people astray right and left.’ He struck one of his hands on the other and then said, ‘Take care lest you destroy the ayat of stoning so that one will say, “We do not find two hadds in the Book of Allah.” The Messenger of Allah, may Allah bless him and grant him peace, stoned, so we have stoned. By He in Whose Hand my self is, had it not been that people would say that Umar ibn al-Khattab has added to the Book of Allah ta-ala, we would have written it, “The full-grown man and the full-grown woman, stone them absolutely.” We have certainly recited that.'”
Malik said, “Yahya ibn Said said, Said ibn al-Musayyab said, ‘Dhu’l-Hijja had not passed before Umar was murdered, may Allah have mercy on him.'”
Yahya said that he had heard Malik say, “As for his words ‘The full-grown man and the full-grown woman’ he meant, ‘The man and the woman who have been married, stone them absolutely.'”