Sahih Muslim Book 20, Hadith Number 4589.

Chapter : It is good on the part of a leader to take the oath of allegiance from his forces when intending to fight, and an account of the allegiance of God’s pleasure under the tree.

The tradition has been narrated on the authority of Sa’id b. Musayyib who learnt it from his father. The latter said: I had seen the tree. When I came to the spot afterwards, I could not recognise it.