Sahih Muslim 1856 d
The Book on Government
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
It has been narrated (through a different chain of transmitters) on the authority of Abu Zubair who heard Jabir being questioned as to whether the Prophet (ﷺ) took the oath of fealty at Dhu'l-Hulaifa. He said: No! But he offered his prayers at that place, and he administered the oath of fealty nowhere except near the tree in (the plain oo Hudaibiya. Ibn Juraij said that he was informed by Abu Zabair who heard Jabir b. Abdullah say: The Prophet (ﷺ) prayed over the well at Hudaibiya (as a result of which its scanty water rose up and increased so as to be sufficient for the 1400 or 1500 men who had encamped at the place).