Hadith
أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا الْحَجَّاجُ بْنُ مِنْهَالٍ، قَالَ حَدَّثَنَا هَمَّامٌ، عَنْ قَتَادَةَ، عَنْ أَبِي حَسَّانَ، قَالَ قَالَ عَلِيٌّ مَا عَهِدَ إِلَىَّ رَسُولُ اللَّهِ صلى الله عليه وسلم بِشَىْءٍ دُونَ النَّاسِ إِلاَّ فِي صَحِيفَةٍ فِي قِرَابِ سَيْفِي . فَلَمْ يَزَالُوا بِهِ حَتَّى أَخْرَجَ الصَّحِيفَةَ فَإِذَا فِيهَا " الْمُؤْمِنُونَ تَكَافَأُ دِمَاؤُهُمْ يَسْعَى بِذِمَّتِهِمْ أَدْنَاهُمْ وَهُمْ يَدٌ عَلَى مَنْ سِوَاهُمْ لاَ يُقْتَلُ مُؤْمِنٌ بِكَافِرٍ وَلاَ ذُو عَهْدٍ فِي عَهْدِهِ " .
It was narrated that Abi Hassan said:"Ali said: 'The Messenger of Allah did not tell me anything that he did not tell the people, except what is in a sheet in the sheath of my word.' They did not leave him alone until he brought out the sheet, and in it (were the words): 'The lives of the believers are equal in value, and they hasten to support the asylum granted by the least of them, and they are one against others. But no believer may be killed in return for a disbeliever, nor one with a covenant while his covenant is in effect
Sahih · Abu Ghuddah
Sunan an-Nasa'i 4745 ·Book 45, Hadith 40·Also available on sunnah.com·