From Sahih Bukhari (Hadith 2785):
Narrated Abu Huraira:A man came to Allah's Messenger (ﷺ) and said, "Instruct me as to such a deed as equals Jihad (in reward)." He replied, "I do not find such a deed." Then he added, "Can you, while the Muslim fighter is in the battle-field, enter your mosque to perform prayers without cease and fast and never break your fast?" The man said, "But who can do that?" Abu Huraira added, "The Mujahid (i.e. Muslim fighter) is rewarded even for the footsteps of his horse while it wanders about (for grazing) tied in a long rope
Reference: Bukhari 2785