Sahih Bukhari | Volume 5 | Hadith Number 557

Narrated by Ibn Abbas
When Allah's Apostle and his companions arrived (at Mecca), the pagans said, "There have come to you a group of people who have been weakened by the fever of Yathrib (i.e. Medina)." So the Prophet ordered his companions to do Ramal (i.e. fast walking) in the first three rounds of Tawaf around the Ka'ba and to walk in between the two corners (i.e. the black stone and the Yemenite corner). The only cause which prevented the Prophet from ordering them to do Ramal in all the rounds of Tawaf, was that he pitied them.