Sahih Bukhari Volume 2, Book 26, Hadith Number 672.

Narated By Ibn Abbas : When Allah’s Apostle and his companions came to Mecca, the pagans circulated the news that a group of people were coming to them and they had been weakened by the Fever of Yathrib (Medina). So the Prophet ordered his companions to do Ramal in the first three rounds of Tawaf of the Ka’ba and to walk between the two corners (The Black Stone and Yemenite corner). The Prophet did not order them to do Ramal in all the rounds of Tawaf out of pity for them.