Sahih al-Bukhari 7208
Narrated Salama:
We gave the oath of allegiance to the Prophet (ﷺ) under the tree. He said to me, "O Salama! Will you not 
give the oath of allegiance?" I replied, "O Allah's Messenger (ﷺ)! I have already given the oath of allegiance 
for the first time." He said, (Give it again) for the second time.
حَدَّثَنَا أَبُو عَاصِمٍ، عَنْ يَزِيدَ بْنِ أَبِي عُبَيْدٍ، عَنْ سَلَمَةَ، قَالَ بَايَعْنَا النَّبِيَّ صلى الله عليه وسلم تَحْتَ الشَّجَرَةِ فَقَالَ لِي ‏"‏ يَا سَلَمَةُ أَلاَ تُبَايِعُ ‏"‏‏.‏ قُلْتُ يَا رَسُولَ اللَّهِ قَدْ بَايَعْتُ فِي الأَوَّلِ‏.‏ قَالَ ‏"‏ وَفِي الثَّانِي ‏"‏‏.‏