This excerpt is taken from Sheikh Abu Hamza Abdul Khaliq Siddiqi's book Sahih Fazail-e-Amaal.
It is narrated from Sayyiduna Zaid bin Khalid رضي الله عنه that the Messenger of Allah صلی اللہ علیہ وآلہ وسلم said:
من جهز غازيا فى سبيل الله فقد غزا، ومن خلف غازيا فى سبيل الله بخير فقد غزا .
"Whoever prepares a warrior (ghazi) is as if he himself has fought in jihad, and whoever takes care of the family of a mujahid with goodness and kindness, it is as if he himself has fought in jihad."
Reference: Sahih al-Bukhari, Book of Jihad, Chapter: The virtue of whoever equips a warrior or leaves behind good, Hadith No: 2843 – Sahih Muslim, Book of Leadership, Chapter: The virtue of helping the warrior in the way of Allah, Hadith No: 1895.
From this hadith, it is understood that cooperating with the mujahideen by providing war equipment and other necessities is equivalent to participating in jihad oneself. Similarly, taking care of the needs of the mujahid’s family after he goes for jihad is also a cause of reward and blessing. Moreover, it teaches brotherhood, fraternity, and a sense of responsibility.
