From Sahih Muslim (Hadith 4869):
It has been narrated on the authority of Abu Huraira who said:The Messenger of Allah (ﷺ) 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 Jihad 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 Mujahid returns from Jihad in the way of Allah, the Exalted
Reference: Muslim 4869