Muwatta Imam Malik (Riwayat Yahya)

Yahya said: Imam Malik, may Allah have mercy on him, said that if a person performed tayammum when he did not find water, then he stood for prayer and said the opening takbir, and then a man passed by who had water with him, he should not break his prayer but should complete it with tayammum. After the prayer, if he finds water, he should perform ablution for future prayers.