“To take my work seriously would be the height of folly.”

Entry in American National Biography

Forrest to his men, 1865. As quoted in May I Quote You, General Forrest? by Randall Bedwell.
1860s

As quoted in Simpson's Contemporary Quotations‎ (1988) by James Beasley Simpson; also quoted in Running on Empty: Meditations for Indispensable Women (1992) by Ellen Sue Stern, p. 235
Paraphrased variants: The most important thing I have learned over the years is the difference between taking one's work seriously and taking one's self seriously. The first is imperative, and the second disastrous.
Take your work seriously, but never yourself.

Source: Pensées Philosophiques (1746), Ch. 5, as quoted in Selected Writings (1966) edited by Lester G. Crocker

undated quotes
Source: Machine in the Studio, Caroline A. Jones, University of Chicago Press, 1996 pp. 197-198

Interview with Rebecca Hardy, Daily Mail 'Weekend' magazine, 27th June 2009

Act 3, sc. 3
Dirty Hands (1948)

Letter to William Thomson (2 January 1851), indicating his early work on what has since become known as Boolean logic.
1850s
Context: I am now about to set seriously to work upon preparing for the press an account of my theory of Logic and Probabilities which in its present state I look upon as the most valuable if not the only valuable contribution that I have made or am likely to make to Science and the thing by which I would desire if at all to be remembered hereafter.