The King's Mirror (Speculum Regale—Konungs Skuggsjá).
Translated from the Old Norse with Introduction and Notes by Laurence Marcellus Larson (New York: Twayne Publishers, Inc., & The American-Scandinavian Foundation, 1917), p. 82:
Train yourself to be as active as possible, though not so as to injure your health. Strive never to be downcast, for a downcast mind is always morbid; try rather to be friendly and genial at all times, of an even temper and never moody. Be upright and teach the right to every man who wishes to learn from you; and always associate with the best men. Guard your tongue carefully; this is good counsel, for your tongue may honor you, but it may also condemn you. Though you be angry speak few words and never in passion; for unless one is careful, he may utter words in wrath that he would later give gold to have unspoken. On the whole, I know of no revenge, though many employ it, that profits a man less than to bandy heated words with another, even though he has a quarrel to settle with him. You shall know of a truth that no virtue is higher or stronger than the power to keep one's tongue from foul or profane speech, tattling, or slanderous talk in any form.
Id., p. 84:
This, too, you must keep constantly in mind, if you wish to be counted a wise man, that you ought never to let a day pass without learning something that will profit you. Be not like those who think it beneath their dignity to hear or learn from others such things even as might avail them much if they knew them. For a man must regard it as great an honor to learn as to teach, if he wishes to be considered thoroughly informed.