Sahih Muslim
The Book of Pilgrimage (Kitab Al-Hajj)
Book Number: 007 / Hadith Number: 2913
Ibn 'Umar (Allah be pleased with them) reported that Umar (Allah be pleased with him) kissed the Stone and said: I am kissing you, whereas I know that you are a stone, but I saw Allah's Messenger (may peace be upon him) kissing you (that Is why I kiss you).