Musnad Abu Ya'la al-Mawsili 155:277

Muhammad ibn al-Muthanna narrated to us, Ibn Abi ‘Adi 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 or seventy-two sects, and the Christians divided into seventy-one or seventy-two sects. And my nation will divide into seventy-three sects."