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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."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدٌ الأَنْصَارِيُّ، قَالَ: حَدَّثَنَا أَبُو مَسْعُودٍ الْجُرَيْرِيُّ قَالَ: سَأَلَ رَجُلٌ الْحَسَنَ عَنْ قِرَاءَةِ بِسْمِ اللهِ الرَّحْمَنِ الرَّحِيمِ؟ قَالَ: تِلْكَ صُدُورُ الرَّسَائِلِ.
sunnah.com