Sayyiduna Uqbah bin Amir al-Juhani (may Allah be pleased with him) said: I was walking with the Messenger of Allah (peace and blessings be upon him) when he (peace and blessings be upon him) said to me: "Say, O Uqbah." I said: What shall I say? Then he fell silent. I said to myself: If he repeats it to me, I will say just what he says. He (peace and blessings be upon him) said again: "Say." Then he (peace and blessings be upon him) recited Surah al-Falaq until he completed it. Then the Messenger of Allah (peace and blessings be upon him) said: "No one who asks has ever asked with anything like it, and no one who seeks refuge has ever sought refuge with anything like it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15020