"In the Name of Allah, the All-Merciful, Most Merciful. To the slave of Allah, 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'"  

أَنَّ زَيْدَ بْنَ ثَابِتٍ كَتَبَ بِهَذِهِ الرِّسَالَةِ بِسْمِ اللهِ الرَّحْمَنِ الرَّحِيمِ لِعَبْدِ اللهِ مُعَاوِيَةَ أَمِيرِ الْمُؤْمِنِينَ مِنْ زَيْدِ بْنِ ثَابِتٍ سَلاَمٌ عَلَيْكَ أَمِيرَ الْمُؤْمِنِينَ وَرَحْمَةُ اللهِ فَإِنِّي أَحْمَدُ إِلَيْكَ اللَّهَ الَّذِي لا إِلَهَ إلا هُوَ أَمَّا بَعْدُ  

Abu Mas'ud al-Jurayri related to him that a man asked al-Hasan about reading, "In the Name of Allah, the All-Merciful, Most Merciful." He said, "That should be put at the beginning of letters."  

حَدَّثَنَا أَبُو مَسْعُودٍ الْجُرَيْرِيُّ قَالَ سَأَلَ رَجُلٌ الْحَسَنَ عَنْ قِرَاءَةِ بِسْمِ اللهِ الرَّحْمَنِ الرَّحِيمِ؟ قَالَ تِلْكَ صُدُورُ الرَّسَائِلِ