Tafsir of Az-Zumar 39:11

(Say, "Indeed, I have been commanded that I worship Allah, [being] sincere to Him in [my] religion")

That is, [free from] whatever would impair it, such as polytheism, showing off, and other such things. He, peace and blessings be upon him, was commanded to declare that which he himself was commanded: the sincerity in the worship of Allah, the Almighty and Majestic—which is synonymous with the piety that the believers were commanded to observe. This serves as an exaggeration in urging them toward that which they are obligated, and as a preparation for that which follows it, in which the polytheists are addressed.

The one who issued the command is not explicitly mentioned, because it is definitively understood that it is Allah, the Almighty and Majestic. It is also said: [it is omitted] to indicate that this command is of the nature that should be complied with, regardless of whether it proceeded from Him, the Exalted, or from other than Him, the Glorified.