Musnad Ahmad 30:3696

'Abd al-Samad narrated to us, Hammad narrated to me, from Muhammad ibn 'Amr, from Abu Salamah, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "You will surely follow the ways of those who came before you, span by span, cubit by cubit, and arm-span by arm-span, until even if one of them were to enter the hole of a mastigure (dhab), you would enter it." They said: "O Messenger of Allah, the Jews and the Christians?" He said: "Who else?"