What then is the conclusion of the matter? Love yourself, if that means rational and healthy self-interest. You are commanded to do that. That is the length of life. Love your neighbor as you love yourself. You are commanded to do that. That is the breadth of life. But never forget that there is a first and even greater commandment: “Love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind.” This is the height of life. Only by a painstaking development of all three of these dimensions can you expect to have a complete life.

Martin Luther King, Jr. (1929–1968) American clergyman, civil rights leader, social activist, preacher

Sermon (1963 ed.), “Three Dimensions of a Complete Life,” sec. 3, Strength to Love, ch. 9
    (Source)

For the Biblical text on the commandments mentioned, see here.

This was one of King's favorite sermons, possibly originating in 1952, and he reworked and delivered it (or reprinted it) throughout his life. The length/breadth/height concept was inspired by a sermon by Episcopal bishop Phillips Brooks (1835–1893), "The Symmetry of Life."

Other versions:

Look up beyond your self interest. Look up beyond your concern for humanity. Look up to the very height of life itself and then you find God who makes life complete. We are commanded to love ourselves and we are commanded to love our neighbors as we love ourselves, but still there is a greater commandment which says “Love the Lord thy God …”
[Sermon (1954-01-24), "The Dimensions of a Complete Life," Dexter Avenue Baptist Church, Montgomery, Alabama]

Love yourself, if that means rational, healthy, and moral self-interest. You are commanded to do that. That is the length of life. Love your neighbor as you love yourself. You are commanded to do that. That is the breadth of life. But never forget that there is a first and even greater commandment, "Love the Lord thy God with all thy heart and all thy soul and all thy mind." This is the height of life. And when you do this you live the complete life.
[Sermon (1959), "Dimensions of a Complete Life," The Measure of a Man]

Love yourself, you are commanded to do that. That is the length of life. Love your neighbor as you love yourself, you are commanded to do that. That’s the breadth of life. But never forget that there is a first and even greater commandment: Love the Lord thy God with all thy heart, with all thy soul, and with all thy mind. That is the height of life. And when you do this, you’ll live the complete life.
[Sermon (1960-02-28), "The Three Dimensions of a Complete Life," Friendship Baptist Church, Pasadena, California]

Love yourself if that means rational, healthy, and moral self-interest. You are commanded to do that; that is the length of life. Love your neighbor as you love yourself. You are commanded to do that; that is the breadth of life. But never forget there is a first and even greater commandment: Love the Lord thy God with all thy heart and with all thy soul and with all thy mind. That is the height of life. When an individual does this, he lives a complete life.
[Sermon (1960-12-11), "The Three Dimensions of a Complete Life," Unitarian Church, Germantown, Pennsylvania]

Go out this morning. Love yourself. That means rational and healthy self-interest; you were commanded to do that. That's the length of life. Then follow that, love your neighbor as you love yourself; you're commanded to do that. That's then breadth of life. I'm going to take my seat now by letting you know that there's a first and even greater commandment: Love the Lord thy God with all thy heart, with all thy soul, with all our strength. I think the psychologists all , with all thy personality. And when you do that, you've got the [height] of life, and when you get all three of these together, you can walk and never get weary ...
[Sermon (1967-04-09), "The Three Dimensions of a Complete Life," New Covenant Baptist Church, Chicago (audio)]