It is recommended for the army to swear allegiance to the ruler when intending to fight, and an account of Bay'at Ar-Ridwan beneath the tree
وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَابْنُ، نُمَيْرٍ قَالاَ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ إِدْرِيسَ، ح وَحَدَّثَنَا رِفَاعَةُ بْنُ الْهَيْثَمِ، حَدَّثَنَا خَالِدٌ، - يَعْنِي الطَّحَّانَ - كِلاَهُمَا يَقُولُ عَنْ حُصَيْنٍ، عَنْ سَالِمِ بْنِ أَبِي الْجَعْدِ، عَنْ جَابِرٍ، قَالَ لَوْ كُنَّا مِائَةَ أَلْفٍ لَكَفَانَا كُنَّا خَمْسَ عَشْرَةَ مِائَةً .
It has been narrated on the authority of Jabir who said: If we had been a hundred thousand in number, it (the water) would have sufficed us, but actually we were fifteen hundred.