Musnad Ahmad 30:3116

Waki' narrated to us, he said: Al-A'mash narrated to us, from Abu Salih, from Abu Hurairah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Look at those who are below you, and do not look at those who are above you, for it is more likely that you will not belittle the favor of Allah upon you."