Hadith: May Allah curse the Muhallil (a man who marries an irrevocably divorced woman for the sole purpose of making her lawful for her ex-husband to remarry) and the Muhallal lahu (the ex-husband of the woman who was irrevocably divorced, seeking to remarry her through an unlawful marriage)

عن علي رضي الله عنه أن النبي صلى الله عليه وسلم قال: «لَعَنَ اللهُ المُحَلِّل، والمُحَلَّلَ له».
[صحيح] - [رواه أبو داود والترمذي وابن ماجه وأحمد]

‘Ali (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "May Allah curse the Muhallil and the Muhallal lahu."
[Authentic hadith] - [Narrated by Ibn Majah - Narrated by At-Termedhy - Narrated by Abu Daoud - Narrated by Ahmad]

Explanation

When a wife is divorced for the third time, she is no longer lawful for her husband until she is married to another man, who should consummate marriage with her. Therefore, some people resort to circumventing legal rulings. An ex-husband would arrange with another man to marry his irrevocably divorced woman through a sham marriage and then divorce her, not intending the Shariah-approved marriage thereby, but with the sole purpose of rendering the irrevocably divorced wife lawful to her ex-husband. Due to such circumvention of Islamic rulings, and for the meanness and the lack of a sense of honor involved in this behavior, the Prophet (may Allah's peace and blessings be upon him) forbade this kind of marriage and invoked curse and expulsion of Allah's mercy on the Muhallil and the Muhallah lahu.