Musnad Ahmad 962:154

Abu al-Mughirah narrated to us, Ma'an ibn Rifa'ah narrated to us, Ali ibn Yazid narrated to me, from al-Qasim, from Abu Umamah, who said: We went out with the Messenger of Allah, peace and blessings of Allah be upon him, on one of his military expeditions. He said: A man passed by a cave in which there was some water. He said: He spoke to himself about staying in that cave, for what was in it of water would sustain him, and he would obtain what was around it of herbs, and he would seclude himself from the world. Then he said: If I were to come to the Prophet of Allah, peace and blessings of Allah be upon him, and mention that to him, if he permits me, I will do it, and if not, I will not do it. So he came to him and said: O Prophet of Allah, I passed by a cave in which there is what sustains me of water and herbs, and my soul spoke to me about staying in it and secluding myself from the world. He said: The Prophet, peace and blessings of Allah be upon him, said: "I was not sent with Judaism nor Christianity, but I was sent with the tolerant Hanifiyyah (monotheism). By the One in Whose hand is the soul of Muhammad, a morning journey or an evening journey in the path of Allah is better than the world and what is in it. And the standing of one of you in the row (for battle) is better than his prayer for sixty years."