Sunan an-Nasai Book 45, Hadith 4734

أَخْبَرَنِي مُحَمَّدُ بْنُ الْمُثَنَّى، قَالَ حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا سَعِيدٌ، عَنْ قَتَادَةَ، عَنِ الْحَسَنِ، عَنْ قَيْسِ بْنِ عُبَادٍ، قَالَ انْطَلَقْتُ أَنَا وَالأَشْتَرُ، إِلَى عَلِيٍّ رضى الله عنه فَقُلْنَا هَلْ عَهِدَ إِلَيْكَ نَبِيُّ اللَّهِ صلى الله عليه وسلم شَيْئًا لَمْ يَعْهَدْهُ إِلَى النَّاسِ عَامَّةً قَالَ لاَ إِلاَّ مَا كَانَ فِي كِتَابِي هَذَا ‏.‏ فَأَخْرَجَ كِتَابًا مِنْ قِرَابِ سَيْفِهِ فَإِذَا فِيهِ ‏ "‏ الْمُؤْمِنُونَ تَكَافَأُ دِمَاؤُهُمْ وَهُمْ يَدٌ عَلَى مَنْ سِوَاهُمْ وَيَسْعَى بِذِمَّتِهِمْ أَدْنَاهُمْ أَلاَ لاَ يُقْتَلُ مُؤْمِنٌ بِكَافِرٍ وَلاَ ذُو عَهْدٍ بِعَهْدِهِ مَنْ أَحْدَثَ حَدَثًا فَعَلَى نَفْسِهِ أَوْ آوَى مُحْدِثًا فَعَلَيْهِ لَعْنَةُ اللَّهِ وَالْمَلاَئِكَةِ وَالنَّاسِ أَجْمَعِينَ ‏"‏ ‏.‏

It was narrated that Qais bin 'Ubad said:"Al-Ashtar and I went to 'Ali, may Allah be pleased with him, and said: Did the Prophet of Allah tell you anything that he did not tell to all the people?' He said: 'No, except what is in this letter of mine.' He brought out a letter from the sheath of his sword and it said therein: "The lives of the believers are equal in value, and they are one against others, and they hasten to support the asylum granted by the least of them. But no believer may be killed in return for a disbeliever, nor one with a covenant while his convenant is in effect. Whoever commits an offense then the blame is on himself, and whoever gives sanctuary to an offender, then upon him will be the curse of Allah, the angels and all the people