Musnad Abu Ya'la al-Mawsili 155:138

On the authority of Abu Hurairah, from the Prophet, may Allah bless him and grant him peace, that he said: "The Jews split into seventy-one sects, and the Christians split into seventy-one or seventy-two sects." He said: "One of the two groups is seventy-one, and the other is seventy-two. And my Ummah will split into seventy-three sects." Muhammad ibn 'Amr is in doubt.