Can the reconnoitrer be sent alone?
باب هَلْ يُبْعَثُ الطَّلِيعَةُ وَحْدَهُ
Sahih al-Bukhari, Book of Fighting for the Cause of Allah (Jihaad)(view book)
Sahih al-Bukhari 2847
Narrated Jabir bin 'Abdullah:
When the Prophet called the people (Sadqa, a sub-narrator, said, 'Most probably that happened on the day of Al-Khandaq) Az-Zubair responded to the call (i.e. to act as a reconnoiter). The Prophet) called the people again and Az-Zubair responded to the call. The Prophet then said, "Every prophet had a disciple and my disciple is Zubair bin Al-'Awwam."
حَدَّثَنَا صَدَقَةُ، أَخْبَرَنَا ابْنُ عُيَيْنَةَ، حَدَّثَنَا ابْنُ الْمُنْكَدِرِ، سَمِعَ جَابِرَ بْنَ عَبْدِ اللَّهِ ـ رضى الله عنهما ـ قَالَ نَدَبَ النَّبِيُّ ﷺ النَّاسَ ـ قَالَ صَدَقَةُ أَظُنُّهُ ـ يَوْمَ الْخَنْدَقِ فَانْتَدَبَ الزُّبَيْرُ، ثُمَّ نَدَبَ فَانْتَدَبَ الزُّبَيْرُ، ثُمَّ نَدَبَ النَّاسَ فَانْتَدَبَ الزُّبَيْرُ فَقَالَ النَّبِيُّ ﷺ " إِنَّ لِكُلِّ نَبِيٍّ حَوَارِيًّا، وَإِنَّ حَوَارِيَّ الزُّبَيْرُ بْنُ الْعَوَّامِ ".