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.”[1]

• (Received Pronunciation) IPA^(key): /ə ˌlɪtl̩ bɜːd ˈtəʊld miː/
• (General American) IPA^(key): /ə ˌlɪt(ə)l bɜɹd ˈtoʊld mi/, [-ˌlɪɾ(ə)l-]

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• Hyphenation: a lit‧tle bird told me

a little bird told me

1. (idiomatic, chiefly humorous) I received the information from a source which I am not prepared to disclose.

   Synonyms: rumour has it, word has it

   Let’s just say I know because a little bird told me.

   • [1546, John Heywood, chapter V, in Julian Sharman, editor, The Proverbs of John Heywood. […], London: George Bell and Sons, […], published 1874, →OCLC, part II, page 121:

     […] I did lately heere, / How fleck and his make use their secret haunting, / By one byrd, that in myne eare was late chaunting.

     The spelling has been modernized.]

   • [c. 1596–1599 (date written), William Shakespeare, The Second Part of Henrie the Fourth, […], quarto edition, London: […] V[alentine] S[immes] for Andrew Wise, and William Aspley, published 1600, →OCLC, [Act V, scene v], signature L, recto:

     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, […]]

   • 1780, [Sophia] Lee, The Chapter of Accidents: A Comedy, […], London: […] T[homas] Cadell, […], →OCLC, Act III, scene [ii], page 45:

     VVell, vvell; I had a little bird told me all this—

   • [1833, [Frederick Marryat], chapter I, in Peter Simple. […], volume III, London: Saunders and Otley, […], published 1834, →OCLC, page 6:

     '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.']

   • 1872 July, [George Webbe Dasent], “Three to One: Or Some Passages out of the Life of Amicia Lady Sweetapple. Chapter XLVIII. Lady Sweetapple Sees Lady Charity, and Hears Something of Edith Price.”, in M[ary] E[lizabeth] Braddon, editor, Belgravia: A London Magazine, volume VIII (Second Series; volume XVIII overall), London: Robson and Sons, […], →OCLC, page 104:

     ‘Is Harry Fortescue in love with Florry Carlton?’ asked Lady Charity. ‘A little bird told me he was as good as engaged to her.’

   • 1908, Edgar Wallace, “Chelmsford”, in The Four Just Men, Boston, Mass.: Small, Maynard & Company, published 1920, →OCLC, part II (The Council of Justice), page 287:

     "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.

   • 2012, Anna Elliott, “Saturday 21 May 1814”, in Georgiana Darcy’s Diary: Jane Austen’s Pride and Prejudice Continued, [U.S.A.]: Wilton Press, →ISBN, page 136:

     A little bird told me you were likely to be soon engaged—and to an earl's nephew […]

This phrase is often used more comically than seriously, especially when the source of the information is obvious to both parties but neither is willing to say, or because a custom calls for a pretence of secrecy or surprise.

• hear through the grapevine

• “a little bird told me” under “bird, n.”, in OED Online ⁠, Oxford, Oxfordshire: Oxford University Press, December 2023.
• “a little bird told me, phrase”, in Lexico, Dictionary.com; Oxford University Press, 2019–2022.