Musnad Ahmad 2:279

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Asim al-Ahwal, from ‘Abdullah ibn Sarjis, who said: I saw the one with the small bald spot—meaning ‘Umar ibn al-Khattab—kissing the Stone, and he said: "Verily, I know that you are a stone, but I saw the Messenger of Allah, may Allah bless him and grant him peace, kissing you."