Sahih Muslim Book 20, Hadith Number 4645.

Chapter : The high position reserved by God for mujahids in paradise.

It has been narrated on the authority of Abu Sa’id Khudri that the Messenger of Allah (may peace be upon him) said (to him): Abu Sa’id, whoever cheerfully accepts Allah as his Lord, Islam as his religion and Muhammad as his Apostle is necessarily entitled to enter Paradise. He (Abu Sa’id) wondered at it and said: Messenger of Allah, repeat it for me. He (the Messenger of Allah) did that and said: There is another act which elevates the position of a man in Paradise to a grade one hundred (higher), and the elevation between one grade and the other is equal to the height of the heaven from the earth. He (Abu Sa’id) said: What is that act? He replied: Jihad in the way of Allah ! Jihad in the way of Allah !