Musnad Ahmad 28:177

Hassan narrated to us, Ibn Lahi'ah narrated to us, Yazid ibn Abi Habib narrated to us, from Suwayd ibn Qays, from Abdullah ibn Amr, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "A day of guarding the frontier is better than fasting for a month and standing in prayer during it."