Sahih Muslim 1878 a
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.
حَدَّثَنَا سَعِيدُ بْنُ مَنْصُورٍ، حَدَّثَنَا خَالِدُ بْنُ عَبْدِ اللَّهِ الْوَاسِطِيُّ، عَنْ سُهَيْلِ بْنِ أَبِي،
 صَالِحٍ عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قِيلَ لِلنَّبِيِّ صلى الله عليه وسلم مَا يَعْدِلُ الْجِهَادَ
 فِي سَبِيلِ اللَّهِ عَزَّ وَجَلَّ قَالَ ‏"‏ لاَ تَسْتَطِيعُونَهُ ‏"‏ ‏.‏ قَالَ فَأَعَادُوا عَلَيْهِ مَرَّتَيْنِ أَوْ ثَلاَثًا
 كُلُّ ذَلِكَ يَقُولُ ‏"‏ لاَ تَسْتَطِيعُونَهُ ‏"‏ ‏.‏ وَقَالَ فِي الثَّالِثَةِ ‏"‏ مَثَلُ الْمُجَاهِدِ فِي سَبِيلِ اللَّهِ كَمَثَلِ
 الصَّائِمِ الْقَائِمِ الْقَانِتِ بِآيَاتِ اللَّهِ لاَ يَفْتُرُ مِنْ صِيَامٍ وَلاَ صَلاَةٍ حَتَّى يَرْجِعَ الْمُجَاهِدُ فِي سَبِيلِ
 اللَّهِ تَعَالَى ‏"‏ ‏.‏