Musannaf Ibn Abi Shayba 1:2060

Ibn Idris narrated to us, from Hisham, from al-Hasan, regarding a man who forgets to rinse his mouth: He said: If he has entered into the prayer, then let him proceed. And if he has not entered into the prayer, then let him rinse his mouth and perform istinshaq (snuffing water into the nose).