Musnad Ahmad 482:98

Hassan ibn Musa narrated to us, Shaiban narrated to us, from Yahya, from Muhammad ibn Ibrahim, that Abu Abdullah informed him that Ibn Abis al-Juhani informed him that the Messenger of Allah (peace and blessings of Allah be upon him) said to him: "O Ibn Abis, shall I not inform you of the best thing with which those who seek refuge seek refuge?" He said: I said: "Yes." So the Messenger of Allah (peace and blessings of Allah be upon him) said: "Say: 'I seek refuge in the Lord of mankind' and 'I seek refuge in the Lord of the daybreak'—these two surahs."