The Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, informed me from Ahmad ibn Muhammad, from his father, from al-Husayn ibn al-Hasan ibn Aban, from al-Husayn ibn Saʿid, from ʿUthman, from Samaah, who said: I asked Imam عليه السلام about them both (rinsing the mouth and sniffing water into the nose), and Imam عليه السلام said: "They are part of the Sunnah. If you forget them, there is no obligation upon you to repeat (the ablution)."