Brigham Young, Chapter Four, Item #13

Many have tried to penetrate to the First Cause of all things; but it would be as easy for an ant to number the grains of sand on the earth. It is not for man, with his limited intelligence, to grasp eternity in his comprehension.... It would be as easy for a gnat to trace the history of man back to his origin as for man to fathom the First Cause of all things, lift the veil of eternity, and reveal the mysteries that have been sought after by philosophers from the beginning. What then, should be the calling and duty of the children of men? Instead of inquiring after the origin of Gods -- instead of trying to explore the depths of eternities that have been, that are, and that will be, instead of endeavoring to discover the boundaries of boundless space, let them seek to know the object of their present existence, and how to apply, in the most profitable manner for their mutual good and salvation, the intelligence they possess. Let them seek to know and thoroughly understand things within their reach, and to make themselves well acquainted with the object of their being here, by diligently seeking unto a super-power for information and by the careful study of the best books (DBY, 25).

Provenance:

DBY, 25 reads:
Many have tried to penetrate to the First Cause of all things; but it would be as easy for an ant to number the grains of sand on the earth. It is not for man, with his limited intelligence, to grasp eternity in his comprehension. There is an eternity of life, from which we were composed by the wisdom and skill of Superior Beings. It would be as easy for a gnat to trace the history of man back to his origin as for man to fathom the First Cause of all things, lift the veil of eternity, and reveal the mysteries that have been sought after by philosophers from the beginning. What then, should be the calling and duty of the children of men? Instead of inquiring after the origin of Gods -- instead of trying to explore the depths of eternities that have been, that are, and that will be, instead of endeavoring to discover the boundaries of boundless space, let them seek to know the object of their present existence, and how to apply, in the most profitable manner for their mutual good and salvation, the intelligence they possess. Let them seek to know and thoroughly understand things within their reach, and to make themselves well acquainted with the object of their being here, by diligently seeking unto a super-power for information and by the careful study of the best books. 7:284-5.
DBY, 25 is taken from the Journal of Discourses, October 9, 1859.

Relevant scriptures:

Brian Madsen's occasionally opinionated observations:

(Please read this disclaimer.)
Note that the Journal of Discourses reads superior Power where DBY reads super-power. I don't know why Widtsoe felt impressed to make that change, nor do I know what its significance is.
Brigham makes an interesting Adam-God reference in his discourse. He says:
Here let me state to all philosophers of every class upon the earth, When you tell me that father Adam was made as we make adobies from the earth, you tell me what I deem an idle tale. When you tell me that the beasts of the field were produced in that manner, you are speaking idle worlds devoid of meaning. There is no such thing in all the eternities where the Gods dwell. Mankind are here because they are the offspring of parents who were first brought here from another planet, and power was given them to propagate their species, and they were commanded to multiply and replenish the earth. The offspring of Adam and Eve are commanded to take the rude elements, and, by the knowledge God has given, to convert them into everything required for their life, health, adornment, wealth, comfort, and consolation. Have we the knowledge to do this? We have. Who gave us this knowledge? Our Father who made us; for he is the only wise God, and to him we owe allegiance; to him we owe our lives. He has brought us forth and taught us all we know. We are not indebted to any other power or God for all our great blessings.

In this paragraph, Brigham simultaneously says (a) that mankind was not created from the dust of the earth, but rather was begotten by a celestial being who was brought here from another planet -- clearly a reference to Brigham's beliefs about Adam; and (b) our Father who made us (who, as he just stated, was brought here from another planet -- in other words, Adam) taught us all we know, and "we are not indebted to any other power or God."
Although this reference seems clear enough to me, Brigham is being somewhat oblique here. As David John Buerger writes,
While in no way discarding [Adam-God], Young advanced his doctrine distinctly less emphatically and less frequently than during the previous four years. A very circumspect tone, for example, is quite evident two years later when, after stating once again that "Mankind are here because they are the offspring of parents who were first brought here from another planet, and power was given to them to propagate their species, and they were commanded to multiply and replenish the earth...," Young concluded with a careful double negative: "Adam and Eve are the parents of all pertaining to the flesh and I would not say that they are not also the parents of our spirits."

That final quotation also comes from this discourse, in which Brigham also says:
God has created of one blood all the nations and kingdoms of men that dwell upon all the face of the earth: black, white, copper-coloured, or whatever their colour, customs, or religion, they have all sprung from the same origin; the blood of all is from the same element. Adam and Eve are the parents of all pertaining to the flesh, and I would not say that they are not also the parents of our spirits.
Note also that this discourse of Brigham Young's contains some of his most controversial statements about Blacks and the Priesthood. Without quoting it here, let me point you to the last sizeable paragraph, the third from the end.

Brian C. Madsen