a little bird told me

The etymology is unknown; in English sources, references to birds passing along information to people date back to at least the 16th century (see the 1546 quotation), and in other languages even earlier. For example, Ecclesiastes 10:20 in the Bible, the original Hebrew version of which is dated to 450–180 B.C.E., states according to the King James Version (spelling modernized): “Curse not the king, no not in thy thought, and curse not the rich in thy bed-chamber: for a bird of the air shall carry the voice, and that which hath wings shall tell the matter.”

• "Let’s just say I know because a little bird told me."
• "[…] I did lately heere, / How fleck and his make use their secret haunting, / By one byrd, that in myne eare was late chaunting."
• "I vvil lay ods, that ere this yeere expire, / VVe beare our ciuil ſvvords and natiue fier, / As farre as France, I heard a bird ſo ſing,[…]"
• "VVell, vvell; I had a little bird told me all this—"
• "'Yes,' replies she, 'and what do you care about them?' / 'It's very much I care,' replied I, 'for a little bird has whispered a secret to me.'"
• "‘Is Harry Fortescue in love with Florry Carlton?’ asked Lady Charity. ‘A little bird told me he was as good as engaged to her.’"
• ""Sorry to disturb you," said the Major, "but you're to be transferred to another prison—why, you aren't undressed!" / "No," said Manfred, lazily kicking off the cover, "but I thought the transfer would be earlier." / "How did you know?" / "About the transfer—oh, a little bird told me," said the prisoner, stretching himself."
• "A little bird told me you were likely to be soon engaged—and to an earl's nephew […]"

• rumour has it
• word has it

• hear through the grapevine