Musnad Ahmad 3:79

'Abdullah narrated to us, Suwayd ibn Sa'id narrated to us, in the year twenty-six, Rashdin ibn Sa'd narrated to us, from Zahrah ibn Ma'bad, from Abu Salih, the freed slave of 'Uthman: That 'Uthman said: "O people, set out early, for I am setting out early." So the people set out early. Then he said: "O people, I am telling you a hadith that I have not spoken of since I heard it from the Messenger of Allah, peace and blessings of Allah be upon him, until this day of mine. The Messenger of Allah, peace and blessings of Allah be upon him, said: 'Indeed, a day of guarding the frontier in the way of Allah is better than a thousand days of anything else, so let a man guard the frontier wherever he wishes.'" "Have I conveyed it to you?" They said: "Yes." He said: "O Allah, bear witness."