Musnad Ahmad 528:6

Al-Hasan ibn Siwar Abu al-Ala narrated to us, Layth—meaning Ibn Sa’d—narrated to us, from Mu’awiyah ibn Salih, that ‘Abd al-Rahman ibn Jubayr narrated to him, from his father, from al-Nawwas ibn Sam’an al-Ansari, from the Messenger of Allah (peace and blessings of Allah be upon him), who said: "Allah has struck a parable: a straight path, and on the two sides of the path are two walls, in which are open doors, and over the doors are lowered curtains. At the beginning of the path is a caller who says: 'O people, enter the path all together and do not deviate.' And there is a caller who calls from above the path; whenever a person intends to open any of those doors, he says: 'Woe to you, do not open it, for if you open it, you will enter it.' The path is Islam, the two walls are the limits of Allah, and the open doors are the prohibitions of Allah. That caller at the beginning of the path is the Book of Allah, and the caller from above the path is the admonisher of Allah in the heart of every Muslim."