Tafsir of Ta-Ha 20:91

(They said) in response to Harun, peace be upon him, (we will not cease being) — meaning, we will not stop (devoted to it) — meaning, remaining (until Musa returns to us).

The apparent state of their condition is that they did not make his return—peace be upon him—a definitive limit for ceasing the worship of the calf, as a promise to abandon it upon his arrival. Rather, they meant to see what he would do and what he would say regarding it. It is said that the words of the Samiri: "This is your god and the god of Musa, but he forgot" (88) were fixed in their minds. Thus, they made his return a condition by way of procrastination and delay, concealing the belief that when he returned, he would agree with them in its worship—far be that from him.

This is predicated on the dialogue between them and Harun, peace be upon him, occurring after the aforementioned statement of the Samiri. Thus, "before" would mean "before the return of Musa." It is noted that this answer supports that meaning, for their saying, "We will not cease..." indicates their devotion at the time of his address, whereas they did not worship the calf until after the Samiri’s statement.

Al-Tayyibi said: This response of theirs is of the "foolish style," the opposite of the "wise style," because they said it out of a lack of concern for evident proofs, just as Nimrod said in response to the Khalil (Abraham), peace be upon him, "I give life and cause death." So contemplate this.

Abu Hayyan used this limitation ("until") as proof that the particle lan does not signify perpetuity, for a limitation only occurs where something is contingent, and thus the contingency is removed by it. You know, however, that the one who argues for its signaling perpetuity does not claim it does so in every instance; this is self-evident.

In some reports, it is stated that when they said this, Harun, peace be upon him, withdrew from them with twelve thousand men—those who did not worship the calf. When Musa, peace be upon him, returned and heard the clamor—for they would prostrate when the calf lowed and would not rise until it lowed again, and in another narration, they would dance when it lowed—he said to the seventy who were with him: "This is the sound of the tribulation." When he arrived, he said to his people what he said, and heard from them what they said.