Abdur Rahman ibn Hurmuz al-A'raj said:

Al-Abbas ibn Abdullah ibn al-Abbas married his daughter to Abdur Rahman ibn al-Hakam, and AbdurRahman married his daughter to him. And they made this (exchange) their dower. Mu'awiyah wrote to Marwan commanding him to separate them. He wrote in his letter: This is the shighar which the Messenger of Allah ﷺ has forbidden.

حَدَّثَنَا مُحَمَّدُ بْنُ يَحْيَى بْنِ فَارِسٍ، حَدَّثَنَا يَعْقُوبُ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا أَبِي، عَنِ ابْنِ إِسْحَاقَ، حَدَّثَنِي عَبْدُ الرَّحْمَنِ بْنُ هُرْمُزَ الأَعْرَجُ، أَنَّ الْعَبَّاسَ بْنَ عَبْدِ اللَّهِ بْنِ الْعَبَّاسِ، أَنْكَحَ عَبْدَ الرَّحْمَنِ بْنَ الْحَكَمِ ابْنَتَهُ وَأَنْكَحَهُ عَبْدُ الرَّحْمَنِ ابْنَتَهُ وَكَانَا جَعَلاَ صَدَاقًا فَكَتَبَ مُعَاوِيَةُ إِلَى مَرْوَانَ يَأْمُرُهُ بِالتَّفْرِيقِ بَيْنَهُمَا وَقَالَ فِي كِتَابِهِ هَذَا الشِّغَارُ الَّذِي نَهَى عَنْهُ رَسُولُ اللَّهِ ﷺ .