Musnad Ahmad 30:403

'Abd al-Wahid al-Haddad narrated to us, from Muhammad ibn 'Amr, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Allah, the Almighty and Majestic, said: 'And who is more unjust than one who creates like My creation? So let them create a gnat, or let them create an atom.'"