Is it permissible for a Muslim to say to his brother, "Be truthful to Allah," as an introduction to other words?

This phrase is permissible, and there is nothing apparent in it that prohibits it, because "actions are judged by their intentions." It is a rhetorical question that can have two meanings:

The first: In the sense of an oath or a vow, meaning: "Will you believe me if I swear by Allah?" This meaning takes the rulings of an oath.

The second: An affirmation of speech, meaning: "Just as you believe in Allah, so too what I say is true." The ruling for this is like the ruling for all other permissible verbal affirmations, provided the speaker is truthful.