Battles (Kitab Al-Malahim)

Dawud :: Book 37 : Hadith 4330

Narrated AbuSa'id al-Khudri:

The Prophet (peace_be_upon_him) said: The best fighting (jihad) in the path of Allah is (to speak) a word of justice to an oppressive ruler.

Source materials are from the University of Southern California MSA site
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