Hadith: Whoever equips a fighter in the cause of Allah has indeed taken part in the fight himself; and whoever looks after the dependents of a fighter in his absence properly has indeed taken part in the fight himself

عن زيد بن خالد الجهني رضي الله عنه مرفوعاً: «مَنْ جَهَّز غَازِياً فِي سَبِيلِ الله فَقَد غَزَا، وَمَنْ خَلَّف غَازِياً في أهلِه بخَير فقَد غزَا».
[صحيح] - [متفق عليه]

Zayd ibn Khālid al-Juhani (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Whoever equips a fighter in the cause of Allah has indeed taken part in the fight himself; and whoever looks after the dependents of a fighter in his absence properly has indeed taken part in the fight himself."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

If someone equips a fighter with a riding animal, supplies, and weapons, he will be rewarded the same way as the fighter, because he aided him upon good. If a fighter wanted to go to battle, but was worried that nobody would be there to take care of his family, he could ask someone from the Muslims to help his family. Whoever takes care of a fighter's family will receive the same reward as the Mujāhid himself, because he aided him. Therefore, whoever helps someone in obeying Allah will receive a similar reward. If someone helps a knowledge seeker by buying him books, securing him a house, or offering him a stipend, etc., he will receive the same reward without any decrease in the student's reward.