Musnad Abu Ya'la al-Mawsili 155:70

Al-Harith ibn Surayj narrated to us, al-Nadr ibn Shumayl narrated to us, from Muhammad ibn ‘Amr, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The Jews divided into seventy-one sects, and the Christians divided into seventy-one or seventy-two sects. And my nation will divide into seventy-three sects."