Ramban on Leviticus 16:34/רמב"ן על ויקרא ט״ז:ל״ד

1AND HE DID AS THE ETERNAL COMMANDED MOSES. This means that Aaron fulfilled all that he was commanded, and he was careful all his life not to enter the Holy of Holies except on the Day of Atonement in order to bring his offerings, according to all that G-d commanded Moses.