al-Sharif al-Radi (d. 406 AH) · Thaqalayn library edition, reproduced verbatim
فَلَمَّا قَرَأَ زِيَادٌ الْكِتَابَ قَالَ: شَهِدَ بِهَا وَرَبِّ الْكَعْبَةِ، وَلَمْ تَزَلْ فِي نَفْسِهِ حَتَّى ادَّعَاهُ مُعَاوِيَةُ.
(as-Sayyid ar-Raḍī says:) When Ziyād read this letter he said, “By Allāh he has testified to it.” This point remained in his mind till Mu‘āwiyah claimed him (as his brother by his father).
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