Hallel Rosh Chodesh

 Taanis (28b) Rav happened to come to Babylonia, where he saw that they were reciting hallel on a New Moon. Unfamiliar with this practice, he thought to stop them, as he assumed that they were reciting hallel unnecessarily. Once he saw that they were omitting portions, he said: I can learn from this that they are maintaining the custom of their forefathers, i.e., they know that it is a custom, not an obligation.