Language:
كتاب الجهاد والسير
56  
Fighting for the Cause of Allah (Jihaad)
(183)
Chapter: To take over the leadership of the army during a battle when there is danger
(183)
باب مَنْ تَأَمَّرَ فِي الْحَرْبِ مِنْ غَيْرِ إِمْرَةٍ إِذَا خَافَ الْعَدُوَّ
Sahih al-Bukhari 3063

Narrated Anas bin Malik:

Allah's Messenger (ﷺ) delivered a sermon and said, "Zaid received the flag and was martyred, then Ja`far took it and was martyred, then `Abdullah bin Rawaha took it and was martyred, and then Khalid bin Al-Walid took it without being appointed, and Allah gave him victory." The Prophet (ﷺ) added, "I am not pleased (or they will not be pleased) that they should remain (alive) with us," while his eyes were shedding tears.

حَدَّثَنَا يَعْقُوبُ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا ابْنُ عُلَيَّةَ، عَنْ أَيُّوبَ، عَنْ حُمَيْدِ بْنِ هِلاَلٍ، عَنْ أَنَسِ بْنِ مَالِكٍ ـ رضى الله عنه ـ قَالَ خَطَبَ رَسُولُ اللَّهِ صلى الله عليه وسلم فَقَالَ ‏ "‏ أَخَذَ الرَّايَةَ زَيْدٌ فَأُصِيبَ، ثُمَّ أَخَذَهَا جَعْفَرٌ فَأُصِيبَ، ثُمَّ أَخَذَهَا عَبْدُ اللَّهِ بْنُ رَوَاحَةَ فَأُصِيبَ، ثُمَّ أَخَذَهَا خَالِدُ بْنُ الْوَلِيدِ عَنْ غَيْرِ إِمْرَةٍ فَفُتِحَ عَلَيْهِ، وَمَا يَسُرُّنِي ـ أَوْ قَالَ مَا يَسُرُّهُمْ ـ أَنَّهُمْ عِنْدَنَا ‏"‏‏.‏ وَقَالَ وَإِنَّ عَيْنَيْهِ لَتَذْرِفَانِ‏.‏
Reference : Sahih al-Bukhari 3063
In-book reference : Book 56, Hadith 268
USC-MSA web (English) reference : Vol. 4, Book 52, Hadith 298
  (deprecated numbering scheme)