Sahih Muslim 36
Sahih Muslim · Book 1: The Book of Faith · Clarifying the number of branches of faith, the best and the least of them, the virtue of modesty (Al-Haya') and the fact that it is part of faith
Zuhri has narrated this hadith with the addition of these words:
حَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ ، حَدَّثَنَا عَبْدُ الرَّزَّاقِ ، أَخْبَرَنَا مَعْمَرٌ ، عَنِ الزُّهْرِيِّ ، بِهَذَا الإِسْنَادِ وَقَالَ مَرَّ بِرَجُلٍ مِنَ الأَنْصَارِ يَعِظُ أَخَاهُ .
He (the Holy Prophet) happened to pass by a mass of Ansar who was instructing his brother (about modesty).