little bird told one, a - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
A source one cannot or will not identify gave this information, as in How did you learn they were getting a divorce?—Oh, a little bird told me. Versions of this idiom date from ancient times and appear in numerous proverb collections.