سنن أبي داود

Narrated Abdullah ibn Khubayb:

We went out one rainy and intensely dark night to look for the Messenger of Allah (ﷺ) to lead us in prayer, and when we found him, he asked: Have you prayed?, but I did not say anything. So he said: Say, but I did not say anything. He again said: Say, but I did not say anything. He then said: Say. So I said: What am I to say? He said: Say: "Say, He is Allah, One," and al-Mu'awwadhatan three times in the morning and evening; they will serve you for every purpose.