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 قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا حَاتِمٌ، - يَعْنِي ابْنَ إِسْمَاعِيلَ - عَنْ يَزِيدَ بْنِ أَبِي، عُبَيْدٍ مَوْلَى سَلَمَةَ بْنِ الأَكْوَعِ قَالَ قُلْتُ لِسَلَمَةَ عَلَى أَىِّ شَىْءٍ بَايَعْتُمْ رَسُولَ اللَّهِ صلى الله عليه وسلم يَوْمَ الْحُدَيْبِيَةِ قَالَ عَلَى الْمَوْتِ .
It has been narrated on the authority of Yazid b. Abu Ubaid (the freed slave of Salama b. al-Akwa') who said: 1 asked Salama as to what effect he had sworn fealty to the Messenger of Allah (ﷺ) on the Day of Hudaibiya. He said: To the effect that we will die fighting.