Musnad Ahmad 30:1221

Hajjaj narrated to us, Ibn Jurayj informed me, Ziyad ibn Sa'd informed me, from Muhammad ibn Zayd ibn al-Muhajir ibn Qunfudh, from Sa'id ibn Abi Sa'id al-Maqburi, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him), who said: "By the One in whose hand is my soul, you will surely follow the ways of those who were before you, span by span, cubit by cubit, and arm-span by arm-span, until even if they entered the hole of a lizard, you would enter it." They said: "Who are they, O Messenger of Allah? The People of the Book?" He said: "Who else?"