Once on Succos, a heavy rain began to fall. Rabbi Pinchas of Koritz was deeply pained that he could not dwell in the Sukkah. He wished to fast so the rain would stop, but fasting is forbidden on Yomtov (and some say it was Chol HaMoed, when fasting is likewise prohibited).
Reb Pinchas proclaimed: "Master of the Universe, I hold trust before You; therefore, I take upon myself to fast immediately after Yomtov so that the rain may cease!" The rain stopped immediately, and after Yomtov, he fulfilled his pledge and fasted.