Exodus 2:21 - Ellicott's Commentary On The Whole Bible

Moses was content to dwell with the man. — Reuel must have been so pleased with the manner and appearance of Moses that he invited him to take service with him — perhaps to share his tent. Moses consented, and in course of time took to wife Zipporah, one of Reuel’s daughters. Marriage with the Midianites was allowed, even under the Law. It has been conjectured that Reuel might have communicated to Moses traditions, or even documents concerning their common ancestor, Abraham, and his family. But there is nothing to indicate the use of letters at this early date by the Midianites.