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.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُصَفَّى، حَدَّثَنَا ابْنُ أَبِي فُدَيْكٍ، قَالَ أَخْبَرَنِي ابْنُ أَبِي ذِئْبٍ، عَنْ أَبِي أَسِيدٍ الْبَرَّادِ، عَنْ مُعَاذِ بْنِ عَبْدِ اللَّهِ بْنِ خُبَيْبٍ، عَنْ أَبِيهِ، أَنَّهُ قَالَ خَرَجْنَا فِي لَيْلَةِ مَطَرٍ وَظُلْمَةٍ شَدِيدَةٍ نَطْلُبُ رَسُولَ اللَّهِ صلى الله عليه وسلم لِيُصَلِّيَ لَنَا فَأَدْرَكْنَاهُ فَقَالَ " أَصَلَّيْتُمْ " . فَلَمْ أَقُلْ شَيْئًا فَقَالَ " قُلْ " . فَلَمْ أَقُلْ شَيْئًا ثُمَّ قَالَ " قُلْ " . فَلَمْ أَقُلْ شَيْئًا ثُمَّ قَالَ " قُلْ " . فَقُلْتُ يَا رَسُولَ اللَّهِ مَا أَقُولُ قَالَ " { قُلْ هُوَ اللَّهُ أَحَدٌ } وَالْمُعَوِّذَتَيْنِ حِينَ تُمْسِي وَحِينَ تُصْبِحُ ثَلاَثَ مَرَّاتٍ تَكْفِيكَ مِنْ كُلِّ شَىْءٍ " .
sunnah.com