Rashi on Exodus 24:13/רש"י על שמות כ״ד:י״ג

1ויקם משה ויהושע משרתו AND MOSES ROSE AND HIS MINISTER JOSHUA — I am not sure in what capacity Joshua appears here, but I think that as a disciple he was accompanying the teacher as far as the place where the bounds of the mountain were marked out, whence onward he was not permitted to proceed. From that point ויעל משה MOSES alone ASCENDED אל הר האלהים THE MOUNTAIN OF GOD, whilst Joshua pitched his tent there and stayed there during the whole forty days which Moses spent on the mountain. For thus we find that when Moses came down from the mountain it states, (Exodus 32:17) “and Joshua heard the voice of the people that they shouted” — from which we may infer that he was not with them in the camp.