It is narrated from Sayyiduna Uqbah bin Amir (may Allah be pleased with him): During a journey I was walking ahead of the she-camel of the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "O Uqbah! Shall I not teach you the two best surahs that are recited?" Then he (peace and blessings be upon him) taught me "Say: I seek refuge in the Lord of daybreak" [Surah al-Falaq: 1] and "Say: I seek refuge in the Lord of mankind" [Surah al-Nas: 1]. He (peace and blessings be upon him) sensed that I was not very pleased, so in the morning, when he (peace and blessings be upon him) dismounted to lead the people in the morning prayer, he recited these same two surahs in the prayer. When the Messenger of Allah (peace and blessings be upon him) finished the prayer, he turned towards me and said: "O Uqbah! What do you think about these surahs?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4048