Sahih al-Bukhari 2991
Fighting for the Cause of Allah (Jihaad)
The recitation of Takbir (Allahu Akbar) in the war
Narrated Anas: The Prophet (ﷺ) reached Khaibar in the morning, while the people were coming out carrying their spades over their shoulders. When they saw him they said, "This is Muhammad and his army! Muhammad and his army!" So, they took refuge in the fort. The Prophet (ﷺ) raised both his hands and said, "Allahu Akbar, Khaibar is ruined, for when we approach a nation (i.e. enemy to fight) then miserable is the morning of the warned ones." Then we found some donkeys which we (killed and) cooked: The announcer of the Prophet (ﷺ) announced: "Allah and His Messenger forbid you to eat donkey's meat." So, all the pots including their contents were turned upside down.