Excerpt from 'The Message':
The Archangel Jibreel informed the Prophet about the wicked plan of the unbelievers and asked him to migrate. The Prophet ordered Ali to sleep in his bed, and pass through the terrible ordeal for the safety of Islam, so that the unbelievers might not think that the Prophet had left, but should remain under the impression that he was in the house. Thus Ali remained contented with remaining in the house, so that he himself could move freely through the streets of Makkah and in its precincts.
The advantage underlying this plan was that the enemies kept guarding the Prophet's house only, and he himself gained time to move to a place of safety.
Now we should see as to who offers to sleep in the Prophet's bed and to sacrifice his own life for him? You will naturally say: 'He who believed in him first and whirled round him from the day of his appointment to the prophetic mission just as a moth whirls round a candle'. It is he who should make sacrifice in this path and that self-sacrificing person is Ali.
The Prophet, therefore, turned to Ali and said: 'Sleep in my bed tonight and cover yourself with the green sheet which I use during sleep, because the enemies have conspired to kill me and it is, therefore, necessary that I should migrate to Yathrib.'
Ali occupied the bed of the Prophet early in the night. When three-fourth of the night had passed, forty persons encircled the house and peeped into it through a hole. They saw the condition of the house to be as usual and thought that the man who was sleeping in the bedroom was the Prophet himself.
The Prophet decided to leave the house when the enemies had surrounded it from all sides and were keeping perfect vigil. Almighty Allah willed to save the great leader of Islam from the clutches of these mean people. The Prophet recited those verses of Surah Yasin which suited his condition at that time and after having read up to the verse:
'so that they cannot see'
He came out of the house immediately and went to the appointed place. It is not clear as to how the Prophet succeeded in crossing the blockade and why the besiegers could not see him.