Hadith
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، أَخْبَرَنَا مَالِكٌ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم نَهَى عَنِ الشِّغَارِ، وَالشِّغَارُ أَنْ يُزَوِّجَ الرَّجُلُ ابْنَتَهُ عَلَى أَنْ يُزَوِّجَهُ الآخَرُ ابْنَتَهُ، لَيْسَ بَيْنَهُمَا صَدَاقٌ.
Narrated Ibn `Umar:Allah's Messenger (ﷺ) forbade Ash-Shighar, which means that somebody marries his daughter to somebody else, and the latter marries his daughter to the former without paying Mahr
Sahih
Sahih al-Bukhari 5112 ·Book 67, Hadith 49·Also available on sunnah.com·