Sayyiduna Umar (may Allah be pleased with him) came to the Black Stone, kissed it and said: "I know that you are a stone; you can neither benefit nor harm. Had I not seen the Messenger of Allah (peace and blessings be upon him) kissing you, I would not have kissed you."
(b) In the narration of Ya'la (bin Ubayd) the wording is: I saw Sayyiduna Umar (may Allah be pleased with him) face the Black Stone, then he said: "By Allah, I know that you are a stone. Had I not seen the Messenger of Allah (peace and blessings be upon him) kissing you, I would not have kissed you." Then he went forward and kissed it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9219