Tafsir of Al-A'raf 7:21

He swore an oath to them both. The reciprocal form (mufā‘alah) is used here for emphasis, because whoever competes with another in an action exerts himself in it, so it was used for its implication. It is said that the reciprocal form is used in its literal sense, and that the oath occurred from both sides, but its object differed: he swore to them regarding his sincerity, and they swore to him regarding their acceptance.

This is critiqued by the argument that it would only hold if the reciprocity existed without mentioning the object of the oath—namely, the advice. Since it is mentioned, it does not hold unless it is said that accepting advice was termed "advice" for the sake of parallelism and correspondence, as was said regarding His saying, "And We made an appointment with Moses," where Moses' commitment to fulfill and attend the appointment was itself called an appointment, thus attributing the expression to the reciprocal form.

It is also said that they asked him, "Do you swear by Allah, the Exalted, that you are indeed among the sincere advisers?" and he swore to them, so that was considered a reciprocal act. According to this, as Ibn al-Munayyir stated, the speech involves laff (folding/grouping), as Adam and Eve (peace be upon them) would not swear by the act of speaking, but by the act of addressing. It is also said that it is closer to the concept of dominance (taghlīb), and it is said that there is no need for such interpretations, as the meaning may simply be that he swore to them by saying to them, "Indeed, I am to you both among the sincere advisers."