Nahj al-Balāgha
Hadith 1414فَلَمَّا 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).