Source identified1
“A liar will not be believed, even when he speaks the truth.”
Repeated deception damages credibility beyond the original lie: once listeners have learned to expect false alarms, even a later truthful warning can fail to restore belief in time.
Source note: Joseph Jacobs, The Fables of Aesop, “The Shepherd Boy and the Wolf,” Project Gutenberg #28. The wise man of the village gives the moral after the boy’s repeated false cries cause the villagers to ignore his real warning.