Looking for a specific ao3 fic
It was one of those fics where after getting out of Hydra, Bucky essentially thinks Steve is his handler, or at least in charge. He starts getting nervous that he isn't getting any sort of discipline or punishment. When Steve tries to explain that he'll never be physically punished again and that he can even leave if he wants to, they have a miscommunication and Bucky thinks Steve will get rid of him, so when he messes up later (breaks a glass or something?) he basically begs Steve to punish him rather than send him away. Steve is horrified and they have to take a while to get things straight, and at one point, Steve takes the string from his shorts and ties it around Bucky's wrist as a sort of promise.
There's a second chapter where down the line, Bucky is a lot more healed and mentally there, and shows Steve that he kept the string.