Kiddushin 33 - Standing Up in Front of a Sage

"Before an old man you shall rise" - refers to a sage of any age, or to any man or woman older than 70, but not to a wicked and ignorant person. The Torah continues, "Fear your God," - meaning that one should not avert his eyes pretending that he did not see the sage.

One is not obligated to suffer monetary loss by standing up in front of a sage; even so, one was obligated to stand up in front of a person who was bringing first fruit to Jerusalem. Why is this? Because a mitzvah in its proper time is especially beloved.