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كتاب الجهاد والسير
56  
Fighting for the Cause of Allah (Jihaad)
(103)
Chapter: Concealing the true destination of a Ghazwa
(103)
باب مَنْ أَرَادَ غَزْوَةً فَوَرَّى بِغَيْرِهَا، وَمَنْ أَحَبَّ الْخُرُوجَ يَوْمَ الْخَمِيسِ
Sahih al-Bukhari 2948

Narrated Ka`b bin Malik:

Whenever Allah's Messenger (ﷺ) intended to carry out a Ghazwa, he would use an equivocation to conceal his real destination till it was the Ghazwa of Tabuk which Allah's Messenger (ﷺ) carried out in very hot weather. As he was going to face a very long journey through a wasteland and was to meet and attack a large number of enemies. So, he made the situation clear to the Muslims so that they might prepare themselves accordingly and get ready to conquer their enemy. The Prophet (ﷺ) informed them of the destination he was heading for.

وَحَدَّثَنِي أَحْمَدُ بْنُ مُحَمَّدٍ، أَخْبَرَنَا عَبْدُ اللَّهِ، أَخْبَرَنَا يُونُسُ، عَنِ الزُّهْرِيِّ، قَالَ أَخْبَرَنِي عَبْدُ الرَّحْمَنِ بْنُ عَبْدِ اللَّهِ بْنِ كَعْبِ بْنِ مَالِكٍ، قَالَ سَمِعْتُ كَعْبَ بْنَ مَالِكٍ ـ رضى الله عنه ـ يَقُولُ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم قَلَّمَا يُرِيدُ غَزْوَةً يَغْزُوهَا إِلاَّ وَرَّى بِغَيْرِهَا، حَتَّى كَانَتْ غَزْوَةُ تَبُوكَ، فَغَزَاهَا رَسُولُ اللَّهِ صلى الله عليه وسلم فِي حَرٍّ شَدِيدٍ، وَاسْتَقْبَلَ سَفَرًا بَعِيدًا وَمَفَازًا، وَاسْتَقْبَلَ غَزْوَ عَدُوٍّ كَثِيرٍ، فَجَلَّى لِلْمُسْلِمِينَ أَمْرَهُمْ، لِيَتَأَهَّبُوا أُهْبَةَ عَدُوِّهِمْ، وَأَخْبَرَهُمْ بِوَجْهِهِ الَّذِي يُرِيدُ‏.‏
Reference : Sahih al-Bukhari 2948
In-book reference : Book 56, Hadith 160
USC-MSA web (English) reference : Vol. 4, Book 52, Hadith 198
  (deprecated numbering scheme)