An-Naml: (79) So rely upon Allah...

The fa (so) in His saying, "So rely upon Allah," is for the purpose of ordering the command based on what has been mentioned of His affairs—Exalted is He—for they necessitate reliance upon Him and call for the command to do so. In mentioning Him by the comprehensive name (Allah), there is reinforcement for that; meaning: rely upon Allah, whose state is this, for it necessitates that everyone should rely upon Him and entrust all their affairs to Him—Majestic and Exalted is He.

And His saying, "Indeed, you are upon the clear truth," is an explicit justification for relying upon Him, due to the fact that he—peace and blessings be upon him—is upon the truth that is manifest, or the one that distinguishes between him and falsehood, or between the righteous and the invalid. For his—may Allah bless him and grant him peace—being such is something that necessitates certainty in His protection, His victory, and His support, without a doubt.