R. Hanina b. Dosa prayed for the son of R. Gamliel and foretold the hour when the fever would leave him. It came true and he knew it from the fluency with which he was able to recite the prayer.
Hanina ben Dosa Knows by His Prayer That the Fever Has Broken
Gaster, Exempla no. 167; cf. Berakhot 34b