Tafsir of Ta-Ha 20:89

{ألا يرجع} As for the one who reads it with a rafʿ (nominative case), it is based on the fact that an is the lightened form of the heavy anna. As for the one who reads it with a naṣb (accusative case), it is based on the fact that it is the an that governs the subjunctive mood for verbs.

{من قبل} Meaning: before the Samiri said to them what he said. It is as if, the moment their eyes fell upon him when he emerged from the pit, they were seduced by him and found him pleasing. Thus, before the Samiri could even speak, Aaron (peace be upon him) hastened to say to them: "You have only been tested by it, and indeed, your Lord is the Most Merciful."

{قال يا هارون ما منعك إذ رأيتهم ضلوا * ألا تتبعن أفعصيت أمري}