Tafsir
Tafsir of Taha 90
وَلَقَدْ قَالَ لَهُمْ هَـٰرُونُ مِن قَبْلُ يَـٰقَوْمِ إِنَّمَا فُتِنتُم بِهِۦ ۖ وَإِنَّ رَبَّكُمُ ٱلرَّحْمَـٰنُ فَٱتَّبِعُونِى وَأَطِيعُوٓا۟ أَمْرِى
And Aaron had already told them before [the return of Moses], "O my people, you are only being tested by it, and indeed, your Lord is the Most Merciful, so follow me and obey my order."
Tafsir Ibn Kathir (abridged)
Hafiz Ibn Kathir · English· Passage 90–91
Harun prohibits them from worship of Calf and the Persistence of the Children of Israel in doing so
Allah, the Exalted, informs of Harun's attempt to prohibit them from worshipping the calf and his telling them that this was only a test for them. He told them that their Lord was the Most Beneficent, Who created everything and decreed for everything its just measure. He is the Owner of the Mighty Throne, the One Who does whatever He wants.
فَاتَّبِعُونِى وَأَطِيعُواْ أَمْرِى
(so follow me and obey my order.) Meaning, "Follow me in that which I am commanding you with and leave that which I forbid you from."
قَالُواْ لَن نَّبْرَحَ عَلَيْهِ عَـكِفِينَ حَتَّى يَرْجِعَ إِلَيْنَا مُوسَى
(They said: "We will not stop worshipping it, until Musa returns to us.") meaning, "We will not cease in our worship of this calf until we hear what Musa has to say about it." So they opposed Harun in this matter and they fought against him, nearly killing him.
Tafsir text: quran.com and the House of Islam API. · Taha 20:90–91