Tafsir of As-Saff 61:2

The call addressing faith in His saying, "O you who have believed, why do you say what you do not do?"—according to the interpretation other than the final opinion regarding the occasion of revelation—is apparent. Upon this, it has been said: It is a mockery of those hypocrites and their faith.

"Lim" (Why) is a compound of the preposition lam (for) and the interrogative ma (what). Its alif has been omitted, according to the grammarians, to distinguish between the declarative and the interrogative. It was not reversed, out of eagerness for the answer. It is also said: It is due to their frequent usage together, thus meriting abbreviation (lightening); though proving such frequency is a difficult matter. It is also said: It is due to their convergence in indicating the object of the inquiry. It has been explained that your saying, "Lim (why) did you do [such-and-such]?"—the object of the inquiry is the cause of the action, so it is as if it were composed of the cause and the action. The cause is signified by the lam, and the action is signified by ma, because it is in the sense of "any thing." The sum total provides this, whereas without the letter [the lam], the object being asked about would be the action alone, and this is as you see.

The meaning is: For what reason do you say what you do not do of goodness and righteousness? This is based on the fact that the pivot of the reprimand, in reality, is their failure to act. It was directed toward their speech as a means of highlighting the multiplication of their sin, by clarifying that the reprehensible act is not merely the abandonment of the promised good, but the promise itself—even though they used to consider it a good deed. Had it been said, "Why do you not do what you say?" it would have been understood from it that the reprehensible act was only the abandonment of what was promised.