Musnad Ahmad 30:1961

Aswad ibn Amir narrated to us, Sharik narrated to us, from Umarah ibn al-Qa'qa', from Abu Zur'ah, from Abu Hurayrah, who attributed the hadith [to the Prophet], he said: "And who is more unjust than one who creates a creation like My creation? So let them create a speck like My creation, or a fly, or a grain."