Kissing black stone

 Narrated 'Abis bin Rabia: 'Umar came near the Black Stone and kissed it and said "No doubt, I know that you are a stone and can neither benefit anyone nor harm anyone. Had I not seen Allah's Apostle kissing you I would not have kissed you."

(Bukhari Hadith 1494)

Similar has been narrated about the First Caliph, Abu Bakr (RA). See Suyuti's Tarikh Al-Khulafa 1/35 cf. Ilal al-Darqutni