““We must know our enemy, at least a little.”
“That’s dangerous,” Prufrax said, almost instinctively.
“Yes, it is. What you know, you cannot hate.””

Source: Time Machines Repaired While-U-Wait (2008), Chapter 18 (p. 219)

Source: So Long, and Thanks for All the Fish

1950s, Loving Your Enemies (Christmas 1957)
Context: Another reason why we must love our enemies is that hate scars the soul and distorts the personality. Mindful that hate is an evil and dangerous force, we too often think of what it does to the person hated. This is understandable, for hate bring irreparable damage to its victims. We have seen its ugly consequences in the ignominious deaths brought to six million Jews by a hate-obsessed madman named Hitler, in the unspeakable violence inflicted upon Negroes by blood-thirsty mobs, in the dark horrors of war, and in the terrible indignities and injustices perpetrated against millions of God's children by unconscionable oppressors.
But there is another side which we must never overlook. Hate is just as injurious to the person who hates. Like an unchecked cancer, hate corrodes the personality and eats away its vital unity. Hate destroys a man's sense of values and his objectivity. It causes him to describe the beautiful as ugly and the ugly as beautiful, and to confuse the true with the false and the false with the true.

This is sometimes attributed to Sun Tzu in combination with the above quote, as well as alone, but it too has not been sourced to any published translation of The Art of War, though it is similar in concept to his famous statement in Ch. 3 : "It is said that if you know your enemies and know yourself, you will not be imperiled in a hundred battles..."
Misattributed

Variant translations
If you know others and know yourself, you will not be imperiled in a hundred battles; if you do not know others but know yourself, you win one and lose one; if you do not know others and do not know yourself, you will be imperiled in every single battle.
Know your enemy and know yourself, find naught in fear for 100 battles. Know yourself but not your enemy, find level of loss and victory. Know not thy enemy nor yourself, wallow in defeat every time.
Literal translation: Know [the] other, know [the] self, hundred battles without danger; not knowing [the] other but know [the] self, one win one loss; not knowing [the] other, not knowing [the] self, every battle must [be] lost.
Source: The Art of War, Chapter III · Strategic Attack

Source: Crocodile Tears

Source: Hainish Cycle, (1974), Chapter 9 (p. 300) — from the protagonist’s major speech.

1960s, The Drum Major Instinct (1968)