“Creative work is serious play.”

Source: Psychological Types, or, The Psychology of Individuation (1921), Ch. 1, p. 82
Context: The dynamic principle of fantasy is play, a characteristic also of the child, and as such it appears inconsistent with the principle of serious work. But without this playing with fantasy no creative work has ever yet come to birth. The debt we owe to the play of imagination is incalculable. It is therefore short-sighted to treat fantasy, on account of its risky or unacceptable nature, as a thing of little worth.

Source: The Artist's Way

Art Psalms (2008), The Vast Expanse

Joyce Carol Oates interviews herself (2013)

Speech in Qum, as quoted in Portrait of an Ascetic Despot, Time, January 7, 1980, 2007-02-02 http://www.time.com/time/magazine/article/0,9171,923857,00.html,
Attributed

Onthemat interview

Variants: It should be noted that the games of children are not games, and must be considered as their most serious actions.
For truly it is to be noted, that children's plays are not sports, and should be deemed as their most serious actions.
Book I, Ch. 23
Attributed