And when regret overcame them and they saw that they had gone astray, they said, "If our Lord does not have mercy upon us and forgive us, we will surely be among the losers."
Allah describes the misguidance of those who worshipped the calf that As-Samiri made for them from the ornaments they borrowed from the Copts. He made the shape of a calf with these ornaments and threw in it a handful of dust from the trace of the horse that the Angel Jibril was riding, and the calf seemed to moo. This occurred after Musa went for the appointed term with his Lord, where Allah told him about what happened when he was on Mount Tur. Allah said about His Honorable Self,
((Allah) said: "Verily, We have tried your people in your absence, and As-Samiri has led them astray") 20:85. The scholars of Tafsir have different views over the calf, whether it actually became alive and mooing, or if it remained made of gold, but the air entering it made it appear to be mooing. These are two opinions. Allah knows best. It was reported that when the statue mooed, the Jews started dancing around it and fell into misguidance because they adored it. They said that this, the calf, is your god and the god of Musa, but Musa forgot it! Allah answered them,
The verse 149 speaks of their remorse on this guilt and their repentance,
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 149–150
There was a clever man named Samiri among the Israelites. When Moses left for the mountain, leaving his people in the care of Aaron, Samiri started misleading them. He collected pieces of jewellery from the people and moulded them into the shape of a calf. The sculpting art of the ancient Egyptians was such that when air passed through it, a bellowing sound came out of the calf’s mouth. People are generally awed by strange things. So this ‘miracle’ of the calf led them to believe that there was something sacred about it and they began worshipping it. Thanks to Samiri’s engaging way of speaking, people started gathering around him. His influence increased to such an extent that he soon acquired a large number of followers, except for Aaron and a few of his companions. Nobody dared to openly protest against him. In the face of this mass following, even the protests of Aaron were not strong enough to sway the people and no other member of the community had the courage to speak up. This has always been the reaction of the public from ancient times to the present day. A clever person by his sheer eloquence may draw a huge crowd, even on some non-issue. The public who are attracted to trivialities which are unrelated to the facts do not have the ability to think deeply enough to analyse what is true and what is not. That is why they rally to the side of such a person, thus adding to his popularity and importance. And if anyone attempts to reveal the truth, he is simply ignored in the same way as Aaron was.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.
