Unthinking respect for authority is the greatest enemy of truth.
Autoritätsdusel ist der größte Feind der Wahrheit.
Original text (German)
Source, in full
Letter to Jost Winteler (1901), quoted in The Private Lives of Albert Einstein by Roger Highfield and Paul Carter (1993), p. 79. Einstein had been annoyed that Paul Drude, editor of Annalen der Physik, had dismissed out of hand some criticisms Einstein made of Drude's electron theory of metals.