Musnad Ahmad 27:1294

Husayn ibn Muhammad narrated to us, Sufyan ibn ‘Uyaynah narrated to us, from Yazid ibn Abi Ziyad, from Ibn Abi Layla, from Ibn ‘Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I am the faction of every Muslim."