For thirteen years, Ali bin Abi Talib (pbuh) stood alongside the Prophet (peace be upon him) in the most difficult situations. The thirteen years of hardship was just finishing. It was true that the migration of the Holy Prophet (pbuh) was forced and due to pressure from the Quraysh and the people of Mecca, but it had a bright future. Everyone knew that this migration was the prelude to success, the prelude to victory. It was the exact time when the movement was leaving the period of tribulations and beginning to enter the period of comfort and dignity. It was the very moment when each person is usually trying to get ahead of others in hopes of gaining a position or rank in society. Imam Ali (pbuh) accepted to sleep in the Prophet’s (pbuh) bed, on a dark night, so that the Prophet could leave his house and city.
That night, the death of the person who was sleeping in that bed was almost certain and definite. We cannot say that everyone knew the end of the story at that time because you and I know the story now, and we know that Amir al-Mu'minin (pbuh) was not martyred in that incident. No. The matter is that on a dark night and at a certain place there was a plan to kill someone, it was certain. They said that in order for the Prophet (pbuh) to get out from that place someone had to sleep in his place, so that when the spies looked at him they would think that someone was there. Who was ready to do this? This sacrifice of Imam Ali (pbuh) in itself is an extremely important event, but the timing of it also adds to its significance. When did this happen? It happened when the period of tribulations was supposed to be coming to an end. They were supposed to be going to form a government and start a more comfortable life. The people of Medina - Yathrib - had become believers, and were waiting for the Prophet, everyone knew this. At that moment, this sacrifice was made by Imam Ali (pbuh). The person who took such an important measure certainly had no personal motivations.
Imam Khamenei, April 28, 1989