Musnad Ahmad Hadith 20284

A companion says that once we were on a journey with the Prophet (peace be upon him). Since there were few riding animals, people would take turns riding. On one occasion, it was the turn for the Prophet (peace be upon him) and me to dismount. The Prophet (peace be upon him) came close to me from behind and placed his hands on my shoulders and said, "Recite: Say, I seek refuge with the Lord of the daybreak (Surah Al-Falaq)." I recited these words. In this way, the Prophet (peace be upon him) recited the entire Surah and I recited it along with him. Then, in the same manner, he told me to recite: "Say, I seek refuge with the Lord of mankind (Surah An-Naas)" and recited the entire Surah, which I also memorized. Then the Prophet (peace be upon him) said, "When you pray, recite these two Surahs in your prayer."