Maliks Muwatta Book 20, Hadith Number 116.
Section : Kissing the Corner of the Black Stone when Saluting the Corners.
Yahya related to me from Malik from Hisham ibn Urwa from his father that Umar ibn al-Khattab said to the corner of the Black Stone while he was doing tawaf of the House, “You are only a stone, and if I had not seen the Messenger of Allah, may Allah bless him and grant him peace, kiss you, I would not do so.” Then he kissed it.
Malik said, “I have heard some of the people of knowledge recommending someone doing tawaf of the House to put his hand to his mouth when he takes it from the Yamani corner.”