Sahih Muslim
The Book on Government (Kitab Al-Imara)
Book Number: 020 / Hadith Number: 4636
It has been narrated on the authority of Abu Huraira who said: The Messenger of Allah (may peace be upon him) was asked: What deed could be an equivalent of Jihad in the way of Allah, the Almighty and Exalted? He answered: You do not have the strength to do that deed. The narrator said: They repeated the question twice or thrice. Every time he answered: You do not have the strength to do it. When the question was asked for the third time, he said: One who goes out for Jibad is like a person who keeps fasts, stands in prayer (constantly), (obeying) Allah's (behests contained in) the verses (of the Qur'an), and does not exhibit any lassitude in fasting and prayer until the Mujihid returns from Jihad in the way of Allah, the Exalted.