Tirmidhi #76 records on the authority of Nawwas ibn Sam’an that the messenger of Allah (sallallahu alyhi wa sallam) said:
“Allah has set forth the following as a parable:
There is a road that leads straight to the destination. On either side of the road there is a wall in which there are open doors with curtains hanging on them. From the remote end of the road, a voice calls, ‘proceed straight and do not turn aside’. Whenever someone intends to lift a curtain from the door another voice calls from above. ‘Beware! Do not lift the curtain; otherwise you will be lured inside’.
The prophet (sallallahu alyhi wa sallaam) explained the parable by saying that the straight path is Islam. the walls are the limits imposed by Allah, the open doors are the things that He has prohibited, the voice which calls from the end of the road is the Quran and the voice which calls from above is Allah’s monitor in the heart of every believer”.
It was declared Hasan Gharib by Tirmidhee and sahih by Hakim #245 with Dhahabi agreeing and Al-Albani , Sahih al-Jami’ #3887
Source: Excerpted from the Book “The Spiritual Cure – An Explanation of Suratul Fatihah”, Page 145, Dar us Sunnah Publishers, Birmingham UK.