Language:
كتاب الجهاد والسير
56
Fighting for the Cause of Allah (Jihaad)
(130)
Chapter: The recitation of Takbir (Allahu Akbar) in the war
(130)
باب التَّكْبِيرِ عِنْدَ الْحَرْبِ

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 Apostle forbid you to eat donkey's meat." So, all the pots including their contents were turned upside down.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ، حَدَّثَنَا سُفْيَانُ، عَنْ أَيُّوبَ، عَنْ مُحَمَّدٍ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ صَبَّحَ النَّبِيُّ صلى الله عليه وسلم خَيْبَرَ وَقَدْ خَرَجُوا بِالْمَسَاحِي عَلَى أَعْنَاقِهِمْ، فَلَمَّا رَأَوْهُ قَالُوا هَذَا مُحَمَّدٌ وَالْخَمِيسُ، مُحَمَّدٌ وَالْخَمِيسُ‏.‏ فَلَجَئُوا إِلَى الْحِصْنِ، فَرَفَعَ النَّبِيُّ صلى الله عليه وسلم يَدَيْهِ وَقَالَ ‏ "‏ اللَّهُ أَكْبَرُ، خَرِبَتْ خَيْبَرُ، إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قَوْمٍ فَسَاءَ صَبَاحُ الْمُنْذَرِينَ ‏"‏‏.‏ وَأَصَبْنَا حُمُرًا فَطَبَخْنَاهَا، فَنَادَى مُنَادِي النَّبِيِّ صلى الله عليه وسلم إِنَّ اللَّهَ وَرَسُولَهُ يَنْهَيَانِكُمْ عَنْ لُحُومِ الْحُمُرِ، فَأُكْفِئَتِ الْقُدُورُ بِمَا فِيهَا‏.‏ تَابَعَهُ عَلِيٌّ عَنْ سُفْيَانَ رَفَعَ النَّبِيُّ صلى الله عليه وسلم يَدَيْهِ‏.‏
Reference : Sahih al-Bukhari 2991
In-book reference : Book 56, Hadith 200
USC-MSA web (English) reference : Vol. 4, Book 52, Hadith 234
  (deprecated numbering scheme)