Sahih Bukhari Volume 5, Book 58, Hadith Number 191.

Narated By Khabbaba : I came to the Prophet while he was leaning against his sheet cloak in the shade of the Ka’ba. We were suffering greatly from the pagans in those days. i said (to him). “Will you invoke Allah (to help us)?” He sat down with a red face and said, “(A believer among) those who were before you used to be combed with iron combs so that nothing of his flesh or nerves would remain on his bones; yet that would never make him desert his religion. A saw might be put over the parting of his head which would be split into two parts, yet all that would never make him abandon his religion. Allah will surely complete this religion (i.e. Islam) so that a traveler from Sana to Hadra-maut will not be afraid of anybody except Allah.” (The sub-narrator, Baiyan added, “Or the wolf, lest it should harm his sheep.”)