Sahih Muslim 1859
Sahih Muslim · Book 33: 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
The tradition has been narrated on the authority of Sa'id b. Musayyib who learnt it from his father. The latter said:
وَحَدَّثَنِي حَجَّاجُ بْنُ الشَّاعِرِ ، وَمُحَمَّدُ بْنُ رَافِعٍ ، قَالاَ حَدَّثَنَا شَبَابَةُ ، حَدَّثَنَا شُعْبَةُ ، عَنْ قَتَادَةَ ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ ، عَنْ أَبِيهِ ، قَالَ لَقَدْ رَأَيْتُ الشَّجَرَةَ ثُمَّ أَتَيْتُهَا بَعْدُ فَلَمْ أَعْرِفْهَا.
I had seen the tree. When I came to the spot afterwards, I could not recognise it.