Ja'far bin Abdullah (may Allah have mercy on him) said: (a) I saw Muhammad bin Abbad bin Ja'far kiss the Black Stone and prostrate on it, then he said: "I saw your maternal uncle Ibn Abbas (may Allah be pleased with him) kissing it and prostrating on it."
(b) And Ibn Abbas (may Allah be pleased with him) said: I saw Umar bin al-Khattab (may Allah be pleased with him) kiss it and prostrate on it, and he said: "I saw the Messenger of Allah (peace and blessings be upon him) do this, so I did it."
(c) The wording of the Musnad of Abu Dawud al-Tayalisi is: Then Umar (may Allah be pleased with him) said: "Had I not seen the Prophet (peace and blessings be upon him) kissing you, I would not have kissed you."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9223