Maliks Muwatta Book 41, Hadith Number 20.
Section : What has No Hadd for It.
Malik related to me from Rabia ibn Abi Abd ar-Rahman that Umar ibn al-Khattab spoke about a man who went out with his wife’s slave-girl on a journey and had intercourse with her and then the wife became jealous and mentioned that to Umar ibn al-Khattab. Umar questioned him about it. He said, “She gave her to me.” Umar said, “Bring me a clear proof or I will stone you.” Rabia added, “The wife confessed that she had given her to him.”