Sahih al-Bukhari 5264Book 68, Hadith 14
وَقَالَ اللَّيْثُ عَنْ نَافِعٍ، كَانَ ابْنُ عُمَرَ إِذَا سُئِلَ عَمَّنْ طَلَّقَ ثَلاَثًا قَالَ لَوْ طَلَّقْتَ مَرَّةً أَوْ مَرَّتَيْنِ فَإِنَّ النَّبِيَّ صلى الله عليه وسلم أَمَرَنِي بِهَذَا، فَإِنْ طَلَّقْتَهَا ثَلاَثًا حَرُمَتْ حَتَّى تَنْكِحَ زَوْجًا غَيْرَكَ.
When Ibn 'Umar was asked about person who had given three divorces, he said, "Would that you gave one or two divorces, for the Prophet (ﷺ) ordered me to do so. If you give three divorces then she cannot be lawful for you until she has married another husband (and is divorced by him)."