And know, reader, that an ounce of mirth, with the same degree of grace, will serve God farther than a pound of sadness.

Thomas Fuller (1608-1661) English churchman, historian

The History of the Worthies of England, “Worthies of Hertfordshire,” “Writers” (1662)
    (Source)

Writing of Jeremiah Dike. By the late 19th Century, Fuller's comment had been paraphrased into something simpler, though still attributed to him:

An ounce of cheerfulness is worth a pound of sadness to serve God with.
[Source 1872, 1895, 1867]

This sentiment is not unique to Fuller. In Richard Baxter's A Treatise of Self-Denial (1659), in "A Dialog of Self-Denial" between Flesh and Spirit, Flesh says:

Why should I think of what will be tomorrow?
An ounce of mirth is worth a pound of sorrow.

The second line here may have been a common English aphorism prior to Fuller and Baxter.