It is narrated from Sayyiduna Nawwas bin Sim'an (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Allah Almighty has set forth a parable: there is a straight path, on both sides of which are two walls; in them are open doors, and over those doors hang curtains. At the start of this path a caller is calling: 'O people! All of you walk on this straight path and do not turn aside this way and that.' And a caller calls from above the path, so whenever one of you wants to open any of those (forbidden) doors, he says: 'Woe to you! Do not open it, for if you open it, you will enter it.' Here 'the path' means Islam, 'the curtains' are the limits set by Allah, 'the open doors' are the things Allah has made forbidden, 'the caller at the head of the path' is the Book of Allah (the Qur'an), and 'the caller from above' is the admonisher from Allah present in the heart of every Muslim (the conscience)." This hadith is sahih (authentic) on the condition of Muslim; I do not know of any hidden defect ('illah) in it, but the two Shaykhs (Bukhari and Muslim) did not record it.