Charles Ellicott Commentary Numbers 11:28

"And Joshua the son of Nun, the minister of Moses, one of his chosen men, answered and said, My lord Moses, forbid them." — Numbers 11:28 (ASV)

My lord Moses, forbid them.— The motive which prompted Joshua in making this request appears to have been similar to that which led St. John to forbid the man to cast out devils who did not follow with the Apostles (Mark 9:38–39; Luke 9:49–50). But as the man did not cast out devils in his own name, but in the name of Christ, so in this case Eldad and Medad prophesied by virtue of the spirit which rested upon them from above, of which the Holy Ghost, not Moses, was the giver.

The motives which deterred Eldad and Medad from going to the tent of meeting are unknown. The history teaches the freeness and the sovereignty of the Holy Spirit’s influences, as was also demonstrated later in the case of Cornelius, when the Holy Ghost fell upon him and upon those who were with him before they received baptism, and they spoke with tongues and magnified God (Acts 10:44–48).