Rashi on Exodus 17:10/רש"י על שמות י״ז:י׳

1AND MOSES AND AARON AND HUR — From this we may infer as regards a public fast that three men must pass before the Holy Ark (i. e. must officiate at prayer) — we may infer this because they (the Israelites) were then engaged in fasting (Mekhilta).

2חור HUR was the son of Miriam and her husband was Caleb (Sotah 11b).