This narration was reported to me by Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him - from Ahmad ibn Muhammad, from his father, from Muhammad ibn Yahya, from Muhammad ibn Ali ibn Mahbub. Additionally, I was informed by al-Husayn ibn Ubaydullah from Ahmad ibn Muhammad ibn Yahya, from his father Muhammad ibn Yahya, from Muhammad ibn Ali ibn Mahbub, from al-Abbas, from Hammad ibn Isa, from Hariz, from Zurara, from Muhammad ibn Muslim, who reported from one of the Imams عليه السلام: I asked Imam عليه السلام about a man who started his prayer while in a state of tayammum, prayed one rak‘ah, and then invalidated his state of purification and found water. Imam عليه السلام said: "He should leave, perform ablution, and then continue his prayer from where he left off, the part he prayed with tayammum."