On a day when Hallel is recited (for example Rosh Chodesh) I've heard that one should not (lichatchila) speak between Shemoneh Esrei and Hallel. Rather, as soon as the chazzon finishes chazaras ha'shatz (repetition of the Shemoneh Esrei), he and the congregation should start reciting Hallel.
Is there such an opinion? What is the source for this? Who are the poskim that discuss this? Are there other opinions (that hold one may speak out)? Is there a difference depending on what the person wants to say?