Sahih Bukhari Volume 2, Book 26, Hadith Number 676.

Narated By Nafi’ : Ibn ‘Umar. said, “I have never missed the touching of these two stones of Ka’ba (the Black Stone and the Yemenite Corner) both in the presence and the absence of crowds, since I saw the Prophet touching them.” I asked Nafi’: “Did Ibn ‘Umar use to walk between the two Corners?” Nafi’ replied, “He used to walk in order that it might be easy for him to touch it (the Corner Stone).”