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 ‏.‏

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." Translation Not Available

Book Ref: Sunan An Nisai Book 27 Hadith 3477
Web Ref:  Sunan An Nisai Vol 4 Book 27 Hadith 3507