Sahih Bukhari Volume 5, Book 63, Hadith 3854

حَدَّثَنَا الْحُمَيْدِيُّ، حَدَّثَنَا سُفْيَانُ، حَدَّثَنَا بَيَانٌ، وَإِسْمَاعِيلُ، قَالاَ سَمِعْنَا قَيْسًا، يَقُولُ سَمِعْتُ خَبَّابًا، يَقُولُ أَتَيْتُ النَّبِيَّ صلى الله عليه وسلم وَهْوَ مُتَوَسِّدٌ بُرْدَةً، وَهْوَ فِي ظِلِّ الْكَعْبَةِ، وَقَدْ لَقِينَا مِنَ الْمُشْرِكِينَ شِدَّةً فَقُلْتُ أَلاَ تَدْعُو اللَّهَ فَقَعَدَ وَهْوَ مُحْمَرٌّ وَجْهُهُ فَقَالَ ‏"‏ لَقَدْ كَانَ مَنْ قَبْلَكُمْ لَيُمْشَطُ بِمِشَاطِ الْحَدِيدِ مَا دُونَ عِظَامِهِ مِنْ لَحْمٍ أَوْ عَصَبٍ مَا يَصْرِفُهُ ذَلِكَ عَنْ دِينِهِ، وَيُوضَعُ الْمِنْشَارُ عَلَى مَفْرِقِ رَأْسِهِ، فَيُشَقُّ بِاثْنَيْنِ، مَا يَصْرِفُهُ ذَلِكَ عَنْ دِينِهِ، وَلَيُتِمَّنَّ اللَّهُ هَذَا الأَمْرَ حَتَّى يَسِيرَ الرَّاكِبُ مِنْ صَنْعَاءَ إِلَى حَضْرَمَوْتَ مَا يَخَافُ إِلاَّ اللَّهَ ‏"‏‏.‏ زَادَ بَيَانٌ وَالذِّئْبَ عَلَى غَنَمِهِ‏.

Narrated 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 traveller 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.")