Tafsir of Ta-Ha 20:51

(Fir`awn) said: "What about the generations of old!'' The most correct opinion concerning the meaning of this, is that when Musa informed Fir`awn that his Lord Who sent him is the One Who creates, sustains, ordains and guides, Fir`awn began to argue, using the previous generations as a proof. He was referring to those people of old who did not worship Allah. In other words, "If the matter is as you say, then what happened to those people! They did not worship your Lord. Instead they worshipped other gods besides Him.''