Sahih Bukhari Volume 7, Book 68, Hadith 5266

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

Nafi said: When Ibn Umar was asked about a person who had given three divorces, he said, “Would that you gave one or two divorces, for the Prophet (pbuh), ordered me to do so. If you give three divorces the she cannot be lawful to you until she has married another husband (and is divorced by him).”