It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that Sayyiduna Umar (may Allah be pleased with him) said: Indeed, I kiss you and I know that you are certainly a stone, only because I saw the Messenger of Allah (peace and blessings be upon him) kissing you.
Hadith Referenceسنن دارمي / من كتاب المناسك / 1902
Ja'far bin Abdullah bin Uthman said: I saw Muhammad bin 'Abbad bin Ja'far touching the Black Stone, kissing it, and placing his head upon it. I asked: What is this? He replied: I saw Sayyiduna 'Umar (may Allah be pleased with him) doing this and heard him say: I know that you are indeed a stone, and I do this only because I saw the Messenger of Allah (peace and blessings be upon him) doing it.
Hadith Referenceسنن دارمي / من كتاب المناسك / 1903