Muhammad ibn Ahmad ibn Yahya, from Muhammad ibn Al-Husayn, from Musa ibn Sa’dan, from Al-Husayn ibn Abi Al-Ala, from Al-Muthanna, from Al-Hasan Al-Sayqal, who said: I said to Abu Abdullah عليه السلام: "A man performed tayammum (dry ablution) and then began his prayer. While he was praying, a river passed by him, and he had already completed one rak’ah." Imam عليه السلام said: "He should perform ghusl (ritual purification) and restart the prayer." I said: "What if he had already completed the entire prayer?" Imam عليه السلام said: "He does not need to repeat it."