The sale is based on leaving the issue of people.
الْبَيْعَةُ عَلَى تَرْكِ مَسْأَلَةِ النَّاسِ
Sunan an-Nasa'i al-Kubra, Book of Pledges(view book)
Sunan an-Nasa'i al-Kubra 7735
ʿAmr b. Manṣūr > Abū Mushir > Saʿīd b. ʿAbd al-ʿAzīz > Rabīʿah b. Yazīd > Abū Idrīs al-Khawlānī > Abū Muslim al-Khawlānī > al-Ḥabīb al-Amīn ʿAwf b. Mālik al-Ashjaʿī
"We were with the Messenger of Allah ﷺ and he said: 'Will you not pledge to the Messenger of Allah ﷺ?' And he repeated it three times. So we stretched forth our hands to give our pledge. We said: 'O Messenger of Allah, we are willing to give you our pledge, but on what?' He said: 'That you will worship Allah and not associate anything with him, and (offer) the five daily prayers.' And he said, very quietly: 'And you will not ask the people for anything.'"
أَخْبَرَنَا عَمْرُو بْنُ مَنْصُورٍ قَالَ حَدَّثَنَا أَبُو مُسْهِرٍ قَالَ حَدَّثَنَا سَعِيدُ بْنُ عَبْدِ الْعَزِيزِ عَنْ رَبِيعَةَ بْنِ يَزِيدَ عَنْ أَبِي إِدْرِيسَ الْخَوْلَانِيِّ عَنْ أَبِي مُسْلِمٍ الْخَوْلَانِيِّ قَالَ حَدَّثَنِي الْحَبِيبُ الْأَمِينُ عَوْفُ بْنُ مَالِكٍ الْأَشْجَعِيُّ قَالَ
كُنَّا عِنْدَ رَسُولِ اللهِ ﷺ فَقَالَ «أَلَا تُبَايِعُونَ رَسُولَ اللهِ ﷺ؟» فَرَدَّدَهَا ثَلَاثَ مَرَّاتٍ فَقَدَّمْنَا أَيْدِيَنَا فَبَايَعْنَا فَقُلْنَا يَا رَسُولَ اللهِ قَدْ بَايَعْنَاكَ فَعَلَامَ؟ قَالَ «عَلَى أَنْ تَعْبُدُوا اللهَ لَا تُشْرِكُوا بِهِ شَيْئًا وَالصَّلَوَاتِ الْخَمْسِ وَأَسَرَّ كَلِمَةً خَفِيَّةً لَا تَسْأَلُوا النَّاسَ شَيْئًا»