Musnad Ahmad 30:2696

Yazid narrated to us, Muhammad—meaning Ibn ‘Amr—informed us, from Abu Salamah, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said—meaning—: "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.'"