Rambam, Mishneh Torah, Hilchot Deot, Chapter 6, 1 & 2

1: It is natural for man’s character to be drawn after the thoughts and actions of his friends and associates, and for him to follow the norms of the people of his country. Therefore, one must associate with righteous and be constantly in the company of the wise, so as to learn from their deeds. Conversely, he should keep away from the wicked who walk in darkness, so as not to learn from their deeds. This is what Shlomo said “He who walks with the wise will become wise; he who walks with the foolish will be destroyed (Proverbs 13:20); and what is said “Happy is the man…”(Psalms 1:1) 2: A person who lives in a place where the norms of behavior are evil and the inhabitants do not follow the straight path should move to a place where the people are righteous and follow the ways of the good. If all the places with which he is familiar and of which he hears reports follow improper paths, as in our times, or if he is unable to move to a place where the patterns of behavior are proper, because of [the presence of] bands of raiding troops, or for health reasons, he should remain alone in seclusion, as is said “Sit alone and be quiet” (Eicha 3:28). If they are wicked and sinful and do not allow him to reside there unless he mingle with them and follow their evil behavior, he should go out to caves, thickets, and deserts [rather than] follow the paths of sinners as it states: "Who will give me a lodging place for wayfarers, in the desert." (Jeremiah 9:1)