Tafsir of Ash-Shu`ara' 26:188

He said: "My Lord is the Best Knower of what you do.'' means, `Allah knows best about you, and if you deserve that, He will punish you therewith, and He will not treat you unjustly.' So this is what happened to them -- as they asked for -- an exact recompense. Allah says:

فكذبوه فٲخذهم عذاب يوم الظلة

إنه كان عذاب يوم عظيم