Sahih Muslim
The Book of Pilgrimage (Kitab Al-Hajj)
Book Number: 007 / Hadith Number: 2915
Abis b. Rabi'a reported: I saw 'Umar (Allah'be pleased with him) kissing the Stone and saying: I am kissing you and I know that you are a stone. And if I had not seen Allah's Messenger (may peace be upon him) kissing you, I would not have kissed you.