“Know your own happiness.”

[Power to choose is much bigger than the challenges one faces,says Gaur Gopal Das, https://timesofindia.indiatimes.com/city/raipur/power-to-choose-is-much-bigger-than-the-challenges-one-faces-says-gaur-gopal-das/articleshow/65516324.cms, Times of India, 23 August 2018]

Source: The Happiness Project: Or Why I Spent a Year Trying to Sing in the Morning, Clean My Closets, Fight Right, Read Aristotle, and Generally Have More Fun

I. Asimov: A Memoir (1994)

Source: Genuine Lies

Source: Emma

Fernbank, London, England October, 1971
1970s
Variant: If you want external happiness, it can be an elusive desire. Internal happiness needs only to be revealed. It is not elusive because it is within you. It is your treasure. If you take someone else's treasure, it is stealing, but if you turn to your own, it is not. Happiness is your own treasure because it lies within you.

"Jubal Harshaw" in the first edition (1961); the later 1991 "Uncut" edition didn't have this line, because it was one Heinlein had added when he went through and trimmed the originally submitted manuscript on which the "Uncut" edition is based. Heinlein also later used a variant of this in The Cat Who Walks Through Walls where he has Xia quote Harshaw: "Dr. Harshaw says that 'the word "love" designates a subjective condition in which the welfare and happiness of another person are essential to one's own happiness.'"
Source: Stranger in a Strange Land (1961; 1991)

Source: Hitch-22: A Memoir