a little bird told me

said if you know who gave you the information being discussed but do not want to say who it was

无可奉告，我不说

"How did you know he was leaving?"

"Oh, let's just say a little bird told me."

“你怎么知道他要走了?”

“噢，反正我知道。”

A little bird told me that you had been caught cheating in the English exam.

我听说你英语考试时作弊被逮到了。