Tongue-Tying Curse

Binds the target’s tongue to keep him or her from talking about some specific subject.

References from the canon

• When Hagrid showed up at Hut-on-the-Rock to give Harry his Hogwart's letter, Uncle Vernon "whispered something that sounded like 'Mimblewimble.'" It's possible that Petunia had told him the incantation for the Tongue-Tying Curse which she learned from her sister Lily, and he was trying to use it on Hagrid to stop him  from telling him the truth about James and Lily Potter's deaths.(PS4)
• Arthur Weasley said that Mad-Eye Moody had set up a couple of curses at number twelve, Grimmauld Place in case Snape returned there. The curses were to both "keep him out and bind his tongue if he trie[d] to talk about the place" (DH6).
• Harry, Ron, and Hermione were affected by this upon their arrival at number twelve, Grimmauld Place. Something whooshed over them like cold air, causing their tongues to curl backward on themselves, making it impossible for them to speak, though their tongues soon unravelled again. It was unpleasant, and afterwards Ron made retching noises and (with Hermione) stammered for a while (DH9).
• This curse appears in the book Curses and Counter-Curses (Pm).