Musnad Ahmad 31:799

Rawh narrated to us, Zuhayr ibn Muhammad narrated to us, Zayd ibn Aslam narrated to us, from ‘Ata’ ibn Yasar, from Abu Sa‘id al-Khudri, that the Messenger of Allah (may Allah bless him and grant him peace) said: "You will surely follow the ways of those who were before you, span by span and cubit by cubit, until even if they were to enter the hole of a lizard, you would follow them." We said: "O Messenger of Allah, the Jews and the Christians?" He said: "Who else?"