Someone who writes at the end of a letter, "Peace be upon and the mercy of Allah" and writes so-and-so, son of so-and-so on the 20th of the month

Ibn Abi-Zinad related from his father that he tool this letter from Kharija ibn Zayd and from the great members of the family of Zayd:
"In the Name of Allah, the All-Merciful, Most Merciful. To the slave of Prophet, Mu'awiya, the Amir al-Mu'minin, from Zayd ibn Thabit. Peace be upon you, Amir al-Mu'minin, and the mercy of Allah. I praise Allah to you. There is no god but Him. Following on from that, you asked me about the inheritance of the grandfather and brothers (and he mentioned the letter). We ask Allah for guidance, preservation and firmness in all our affairs. We seek refuge with Allah from being misguided or ignorant or taking on what we have no knowledge of. Peace be upon you, Amir al-Mu'minin, and the mercy of Allah and His blessings and His forgiveness. Wuhayb has written it on Thursday, the 20th Ramadan, 42 (AH)."

حَدَّثَنَا ابْنُ أَبِي مَرْيَمَ، قَالَ: أَخْبَرَنَا ابْنُ أَبِي الزِّنَادِ قَالَ: حَدَّثَنِي أَبِي، أَنَّهُ أَخَذَ هَذِهِ الرِّسَالَةَ مِنْ خَارِجَةَ بْنِ زَيْدٍ، وَمِنْ كُبَرَاءِ آلِ زَيْدٍ: بِسْمِ اللهِ الرَّحْمَنِ الرَّحِيمِ، لِعَبْدِ اللهِ مُعَاوِيَةَ أَمِيرِ الْمُؤْمِنِينَ، مِنْ زَيْدِ بْنِ ثَابِتٍ: سَلاَمٌ عَلَيْكَ أَمِيرَ الْمُؤْمِنِينَ وَرَحْمَةُ اللهِ، فَإِنِّي أَحْمَدُ إِلَيْكَ اللَّهَ الَّذِي لاَ إِلَهَ إِلاَّ هُوَ، أَمَّا بَعْدُ: فَإِنَّكَ تَسْأَلُنِي عَنْ مِيرَاثِ الْجَدِّ وَالإِخْوَةِ، فَذَكَرَ الرِّسَالَةَ، وَنَسْأَلُ اللَّهَ الْهُدَى وَالْحِفْظَ وَالتَّثَبُّتَ فِي أَمْرِنَا كُلِّهِ، وَنَعُوذُ بِاللَّهِ أَنْ نَضِلَّ، أَوْ نَجْهَلَ، أَوْ نُكَلَّفَ مَا لَيْسَ لَنَا بِهِ عِلْمٌ، وَالسَّلاَمُ عَلَيْكَ أَمِيرَ الْمُؤْمِنِينَ وَرَحْمَةُ اللهِ وَبَرَكَاتُهُ وَمَغْفِرَتُهُ. وَكَتَبَ وُهَيْبٌ: يَوْمَ الْخَمِيسِ لِثِنْتَيْ عَشْرَةَ بَقِيَتْ مِنْ رَمَضَانَ سَنَةَ اثْنَيْنِ وَأَرْبَعِينَ.