Rambam's Mussar to Son

Question

Where precisely does the Rambam give mussar to his son about avoiding conflict? This article from Klal Perspectives quotes Kisvei HaRambam, Mussar l’bno Rav Avraham saying

Do not sully yourself with machlokes (strife) that destroys the body, the soul and property, leaving nothing. I have seen the bright blackened, leaders diminished, families broken, princes demoted from their positions, large cities weakened, groups disbanded, pious people lost, the trustworthy erased [and] honored people shamed and disgraced – all as a result of strife. Prophets prophesized, wise men shared their wisdom and philosophers explored and elaborated on the evils of strife, and they all could not truly capture the extent of it. Thus, I urge you to despise it and distance yourself from it and from all those who consider themselves its friends and supporters.

but where can I find that particular text?

Answer 1

An English translation of this letter (slightly different from the one you cite) was published by Leon Stitskin in Tradition 9:4 (Spring 1968) under the title "The Last Will and Testament of Maimonides". It begins on page 80, with the relevant part beginning five lines from the bottom on page 86:

Do not contaminate, moreover, your souls with controversies which consume the body, the soul and property – what else is then left? I have seen the way white is turned black, the refined vulgarized, families torn asunder, rulers removed from their high positions, big cities destroyed, communities deserted, the pious perverted, the faithful spoiled, the honorable disparaged and despised – all because of controversy. The prophets prophesied against it; the wise men poured out their wisdom and the philosophers probed its rationale and underscored the evil of quarrelsomeness, but to no avail. Therefore, resolve in your heart to detest and run away from acrimony and dissassociate yourselves from its companions, well-wishers and neighbors. Remain aloof and stay at a distance even from quarreling relatives lest you suffer from their aberrations.

Note that R. Yitzchak Sheilat in Igrot HaRambam classifies this letter in the category of "definite forgeries", and advances a number of arguments in explanation.