“Introspect daily, detect diligently, negate ruthlessly.”

The Novel of the Future (1969)

This is in fact something an admirer said, which Christie quoted with disapproval in LIFE magazine (14 May 1956), p. 98
Misattributed

How Did Software Get So Reliable Without Proof? Lecture Notes in Computer Science vol 1051 1996 pp. 1-17 : FME '96: Industrial Benefit and Advances in Formal Methods, Third International Symposium of Formal Methods Europe, Co-Sponsored by IFIP WG 14.3, Oxford, UK, March 18-22, 1996, Proceedings.

Space, Time and Gravitation (1920)
Context: It is of interest to inquire what happens when the aviator's speed... approximates to the velocity of light. Lengths in the direction of flight become smaller and smaller, until for the speed of light they shrink to zero. The aviator and the objects accompanying him shrink to two dimensions. We are saved the difficulty of imagining how the processes of life can go on in two dimensions, because nothing goes on. Time is arrested altogether. This is the description according to the terrestrial observer. The aviator himself detects nothing unusual; he does not perceive that he has stopped moving. He is merely waiting for the next instant to come before making the next movement; and the mere fact that time is arrested means that he does not perceive that the next instant is a long time coming.<!--p.26

1820s, Signs of the Times (1829)

Principles of Modern Chemistry (7th ed., 2012), Ch. 1 : The Atom in Modern Chemistry