“That man, I declare, is happy whom nothing makes less strong than he is; he keeps to the heights, leaning upon none but himself; for one who sustains himself by any prop may fall.”
Epistulae Morales ad Lucilium (Moral Letters to Lucilius), Letter XCII: On the Happy Life
Help us to complete the source, original and additional information
Seneca the Younger 225
Roman Stoic philosopher, statesman, and dramatist -4–65 BCRelated quotes

“Man never falls so low, that he can see nothing higher than himself.”
"A Lesson for the Day; or The Christianity of Christ, of the Church, and of Society" in The Dial (October 1940), p. 196.
Context: Every man has at times in his mind the Ideal of what he should be, but is not. This ideal may be high and complete, or it may be quite low and insufficient; yet in all men, that really seek to improve, it is better than the actual character. Perhaps no one is satisfied with himself, so that he never wishes to be wiser, better, and more holy. Man never falls so low, that he can see nothing higher than himself.

“No man is born unto himself alone;
Who lives unto himself, he lives to none.”
Esther (1621), Sec. 1, Meditation 1.

Adv. Prax. 18 http://www.intratext.com/IXT/LAT0788/_P1.HTM
Against Praxeas https://www.newadvent.org/fathers/0317.htm
Original: (la) Igitur unus deus pater, et absque eo alius non est: quod ipse inferens non filium negat sed alium deum: ceterum alius a patre filius non est.

“Man is nothing else but what he makes of himself.”
No Exit (1944)
Variant: A man is what he wills himself to be.
Source: Existentialism and Human Emotions

God and the State (1871; publ. 1882)
Context: A person is strong only when he stands upon his own truth, when he speaks and acts from his deepest convictions. Then, whatever the situation he may be in, he always knows what he must say and do. He may fall, but he cannot bring shame upon himself or his cause. If we seek the liberation of the people by means of a lie, we will surely grow confused, go astray, and lose sight of our objective, and if we have any influence at all on the people we will lead them astray as well — in other words, we will be acting in the spirit of reaction and to its benefit.

Riyadh-as-Saliheen by Imam Al-Nawawi, 647 https://bewley.virtualave.net/riyad4.html
Sunni Hadith

Epistle to Mrs. Higgons (1690), line 79; reported in Hoyt's New Cyclopedia Of Practical Quotations (1922), "Contentment", p. 133-36.