Sunan an-Nasa'i, Volume 4, Book 27, Hadith 3507 (DarusSalam) ⓘ
It was narrated that Ibn 'Umar said: "The Messenger of Allah conducted the procedure of Li'an between a man and his wife, and he separated them and attributed the child to his mother."
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا مَالِكٌ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ لاَعَنَ رَسُولُ اللَّهِ صلى الله عليه وسلم بَيْنَ رَجُلٍ وَامْرَأَتِهِ وَفَرَّقَ بَيْنَهُمَا وَأَلْحَقَ الْوَلَدَ بِالأُمِّ .
also referenced under: Hadith 3477 (Reference) & Book 27, Hadith 89 (In-Book)