Sahih Bukhari Volume 005, Book 057, Hadith Number 053.
Narated By Abu Hazim : A man came to Sahl bin Sad and said, "This is so-and-so," meaning the Governor of Medina, "He is calling 'Ali bad names near the pulpit." Sahl asked, "What is he saying?" He (i.e. the man) replied, "He calls him (i.e. 'Ali) Abu Turab." Sahl laughed and said, "By Allah, none but the Prophet called him by this name and no name was dearer to 'Ali than this." So I asked Sahl to tell me more, saying, "O Abu 'Abbas! How (was this name given to 'Ali)?" Sahl said, "'Ali went to Fatima and then came out and slept in the Mosque. The Prophet asked Fatima, "Where is your cousin?" She said, "In the Mosque." The Prophet went to him and found that his (i.e. Ali's) covering sheet had slipped of his back and dust had soiled his back. The Prophet started wiping the dust off his back and said twice, "Get up! O Abu Turab (i.e. O. man with the dust)."