Musnad Ahmad 2:18

Aswad ibn Amir narrated to us, he said: Zuhayr narrated to us, from Sulayman al-A'mash, Ibrahim narrated to us, from 'Abis ibn Rabi'ah, he said: I saw 'Umar look at the Stone, and he said: "As for by Allah, were it not that I saw the Messenger of Allah, may Allah bless him and grant him peace, kiss you, I would not have kissed you." Then he kissed it.