Sunan an-Nasai Book 38, Hadith 4146

أَخْبَرَنَا عَمْرُو بْنُ يَحْيَى، قَالَ حَدَّثَنَا مَحْبُوبٌ، قَالَ أَنْبَأَنَا أَبُو إِسْحَاقَ، عَنْ سَعِيدٍ الْجُرَيْرِيِّ، عَنْ يَزِيدَ بْنِ الشِّخِّيرِ، قَالَ بَيْنَا أَنَا مَعَ، مُطَرِّفٍ بِالْمِرْبَدِ إِذْ دَخَلَ رَجُلٌ مَعَهُ قِطْعَةُ أُدْمٍ قَالَ كَتَبَ لِي هَذِهِ رَسُولُ اللَّهِ صلى الله عليه وسلم فَهَلْ أَحَدٌ مِنْكُمْ يَقْرَأُ قَالَ قُلْتُ أَنَا أَقْرَأُ فَإِذَا فِيهَا ‏ "‏ مِنْ مُحَمَّدٍ النَّبِيِّ صلى الله عليه وسلم لِبَنِي زُهَيْرِ بْنِ أُقَيْشٍ أَنَّهُمْ إِنْ شَهِدُوا أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنَّ مُحَمَّدًا رَسُولُ اللَّهِ وَفَارَقُوا الْمُشْرِكِينَ وَأَقَرُّوا بِالْخُمُسِ فِي غَنَائِمِهِمْ وَسَهْمِ النَّبِيِّ صلى الله عليه وسلم وَصَفِيِّهِ فَإِنَّهُمْ آمِنُونَ بِأَمَانِ اللَّهِ وَرَسُولِهِ ‏"‏ ‏.‏

It was narrated that Yazid bin Ash-Shikhkhir said:"While I was with Mutarrif in Al-Mirbad, a man came in carrying a piece of leather and said: 'This was written to me by the Messenger of Allah. Is there anyone among you who can read?' I said: 'I can read.' And it was (a letter) from Muhammad the Prophet to Banu Zuhair bin Uqaish, who had testified to Lailah illallah, and that Muhammad is the Messenger of Allah, and had left the idolaters, and had agreed to give the Khumus from their spoils of the Prophet, and wheatever he chose for himself, so they became safe and secure by the covenant of Allah and His Messenger