Tafsir of Al-An'am 6:156

(Lest you say): This is the cause (‘illah) for an omitted [prepositional phrase] indicated by the aforementioned (We sent down), which is the governing agent (‘amil) for it, not the aforementioned [text] itself—contrary to al-Kisa’i—so as to avoid the separation of the governor and the governed by an extraneous element. According to the Kufans, there is an omitted la (i.e., li-an la taqulu [so that you do not say]), and according to the Basrans, there is an omitted genitive noun (mudaf), meaning: "out of fear that you might say."

It has been said: It is possible that it is the object (maf‘ul) of (Fear), a view held by al-Farra’. Or, the implied lam (preposition) could be one of consequence (li-l-‘aqibah), meaning that one of the two statements resulted from Our sending down [the Book], as a consequence results from an action; thus, it serves as a rebuke to them for their remoteness from felicity. The most immediate interpretation is that which was mentioned first: that you might say on the Day of Resurrection, had We not sent it down, "The Book—which speaks the rulings and cuts off the argument—was sent down only..."

(Upon two factions): Two groups existing (before us). They are, as stated by Ibn Abbas and others, the Jews and the Christians. The specificity of the revelation to their two books is because they are the ones among the heavenly books renowned for containing [legal] rulings.

(And indeed, we were): In here is the softened form of anna (that), and the lam that follows is the "discriminating" (fariqah) one, which distinguishes it from the negative in. It is inert (muhmalah), according to what the grammarians have established: that when the softened anna is attached to the lam in one of its two parts and is preceded by a "copyist" (nasikh), it is inert and does not govern [the noun] either explicitly or implicitly, nor [does it affect anything] fixed or omitted. The meaning is: "And that we were (unaware of their study)," meaning their recitation,

(unheeding): Not paying attention, not knowing what it is because it was not in our language, and thus we were unable to receive from it that which contained our salvation. Perhaps they meant by that "Monotheism." It has been said: [It refers to] those rulings mentioned in the words of the Exalted, "(Say: Come, I will recite...)" and so forth, because they are universal to all the children of Adam and do not differ from one era to another. Upon this, the Sheikh al-Islam interpreted the verse, then added: Thus, it becomes clear that this excuse of theirs—despite the fact that they were not commanded with what was in the [two] books—is because they contain the aforementioned rulings that encompass all nations, just as that excuse is cut off by the sending down of the Quran, for it also contains them—not merely [containing] the rest of the laws and rulings.