Musnad Ahmad 937:256

Waki' narrated to us, Mas'udi narrated to us, Abu 'Umar al-Dimashqi informed me, from 'Ubayd ibn al-Khashkhash, from Abu Dharr, who said: I came to the Messenger of Allah (peace and blessings of Allah be upon him) while he was in the mosque, so I sat down. He said: "O Abu Dharr, have you prayed?" I said: No. He said: "Stand and pray." He said: So I stood and prayed, then I sat down. He said: "O Abu Dharr, seek refuge in Allah from the evil of the devils of mankind and the jinn." I said: O Messenger of Allah, do humans have devils? He said: "Yes." I said: O Messenger of Allah, what about the prayer? He said: "The best subject; whoever wishes may do little, and whoever wishes may do much." I said: O Messenger of Allah, what about fasting? He said: "An obligatory duty that suffices, and with Allah is more." I said: O Messenger of Allah, what about charity? He said: "Multiplied manifold." I said: O Messenger of Allah, which of them is best? He said: "The effort of one who has little, or a secret [gift] to a poor person." I said: O Messenger of Allah, which prophet was the first? He said: "Adam." I said: O Messenger of Allah, was he a prophet? He said: "Yes, a prophet spoken to." I said: O Messenger of Allah, how many were the messengers? He said: "Three hundred and some odd, a large multitude." And he said once: "Fifteen." I said: O Messenger of Allah, was Adam a prophet? He said: "Yes, a prophet spoken to." I said: O Messenger of Allah, which of what was revealed to you is the greatest? He said: "The Verse of the Throne (Ayat al-Kursi): 'Allah - there is no deity except Him, the Ever-Living, the Sustainer of [all] existence' [Al-Baqarah: 255]."