“He who knows others is wise. He who knows himself is enlightened.”

Source: Means and Ends of Education (1895), Chapter 1 "Truth and Love"

As quoted in The Works of the Emperor Julian (1923) by Wilmer Cave France Wright, p. 91
General sources

El que no se hallare con ánimo de sufrir apele al retiro de sí mismo, si es que aun a sí mismo se ha de poder tolerar.
Maxim 159 (p. 90)
The Art of Worldly Wisdom (1647)

Source: Aphorisms and Reflections (1901), p. 233

Source: The Monk Who Sold His Ferrari: A Fable About Fulfilling Your Dreams Reaching Your Destiny

Borgi
Paradísarheimt (Paradise Reclaimed) (1960)

Leonard Read Journals, September 6, 1959 https://history.fee.org/leonard-read-journal/1959/leonard-e-read-journal-september-1959

Variant: He who conquers others is strong; He who conquers himself is mighty.

§ IV
1910s, At the Feet of the Master (1911)
Context: These three great crimes you must avoid, for they are fatal to all progress, because they sin against love. But not only must you thus refrain from evil; you must be active in doing good. You must be so filled with the intense desire of service that you are ever on the watch to render it to all around you — not to man alone, but even to animals and plants. You must render it in small things every day, that the habit may be formed, so that you may not miss the rare opportunity when the great thing offers itself to be done. For if you yearn to be one with God, it is not for your own sake; it is that you may be a channel through which His love may flow to reach your fellow-men.
He who is on the Path exists not for himself, but for others; he has forgotten himself, in order that he may serve them. He is as a pen in the hand of God, through which His thought may flow, and find for itself an expression down here, which without a pen it could not have. Yet at the same time he is also a living plume of fire, raying out upon the world the Divine Love which fills his heart.
The wisdom which enables you to help, the will which directs the wisdom, the love which inspires the will — these are your qualifications. Will, Wisdom and Love are the three aspects of the Logos; and you, who wish to enroll yourselves to serve Him, must show forth these aspects in the world.