Language:
كتاب الجهاد والسير
56  
Fighting for the Cause of Allah (Jihaad)
(76)
Chapter: The help of poor and pious men in war
(76)
باب مَنِ اسْتَعَانَ بِالضُّعَفَاءِ وَالصَّالِحِينَ فِي الْحَرْبِ
Sahih al-Bukhari 2896

Narrated Mus`ab bin Sa`d:

Once Sa`d (bin Abi Waqqas) thought that he was superior to those who were below him in rank. On that the Prophet (ﷺ) said, "You gain no victory or livelihood except through (the blessings and invocations of) the poor amongst you."

حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، حَدَّثَنَا مُحَمَّدُ بْنُ طَلْحَةَ، عَنْ طَلْحَةَ، عَنْ مُصْعَبِ بْنِ سَعْدٍ، قَالَ رَأَى سَعْدٌ ـ رضى الله عنه ـ أَنَّ لَهُ فَضْلاً عَلَى مَنْ دُونَهُ، فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏ "‏ هَلْ تُنْصَرُونَ وَتُرْزَقُونَ إِلاَّ بِضُعَفَائِكُمْ ‏"‏‏.‏
Reference : Sahih al-Bukhari 2896
In-book reference : Book 56, Hadith 110
USC-MSA web (English) reference : Vol. 4, Book 52, Hadith 145
  (deprecated numbering scheme)