Al-Kāfi

Hadith 2334
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5ـ عَنْهُ عَنْ عُثْمَانَ بْنِ عِيسَى عَنْ سَمَاعَةَ قَالَ سَأَلْتُ أَبَا عَبْدِ الله (عَلَيهِ السَّلام) عَنِ الرَّجُلِ يَبْدَأُ بِالرَّجُلِ فِي الْكِتَابِ قَالَ لا بَأْسَ بِهِ ذَلِكَ مِنَ الْفَضْلِ يَبْدَأُ الرَّجُلُ بِأَخِيهِ يُكْرِمُهُ.
5. It is narrated from him (narrator of the Hadith above) from ‘Uthman ibn ‘Isa from Sama’a who has said the following: “Once I asked Abu ‘Abdillah (a.s.) about a man who initiates writing to another man. The Imam said, ‘It is not an offense. It is an excellent act when a man initiates honoring his brother (in belief).”’
Source: Shaykh Muḥammad b. Yaʿqūb al-Kulaynī