To take over the leadership of the army during a battle when there is danger
باب مَنْ تَأَمَّرَ فِي الْحَرْبِ مِنْ غَيْرِ إِمْرَةٍ إِذَا خَافَ الْعَدُوَّ
Sahih al-Bukhari, Book of Fighting for the Cause of Allah (Jihaad)(view book)
Sahih al-Bukhari 3063
Narrated Anas bin Malik:
Allah's Apostle 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 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 ﷺ 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.