On the authority of Nawwas ibn Sam’an, the Messenger of Allah, salallahu 'alayhi wa salaam, 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 over 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, salallahu 'alayhi wa salaam, 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 Qur'an and;
The voice which calls from above is Allah’s monitor in the heart of every Believer”.
[at-Tirmidhi #76: It was declared Hasan Gharib by Tirmidhee and Sahih by Hakim #245 with Dhahabi agreeing and Al-Albani ,sahih al-Jami’ #3887]