Sahih al-Bukhari 3776
Narrated Ghailan bin Jarir:
I asked Anas, "Tell me about the name 'Al-Ansar.; Did you call yourselves by it or did Allah call you 
by it?" He said, "Allah called us by it." We used to visit Anas (at Basra) and he used to narrate to us 
the virtues and deeds of the Ansar, and he used to address me or a person from the tribe of Al-Azd and 
say, "Your tribe did so-and-so on such-and-such a day."
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا مَهْدِيُّ بْنُ مَيْمُونٍ، حَدَّثَنَا غَيْلاَنُ بْنُ جَرِيرٍ، قَالَ قُلْتُ لأَنَسٍ أَرَأَيْتَ اسْمَ الأَنْصَارِ كُنْتُمْ تُسَمَّوْنَ بِهِ، أَمْ سَمَّاكُمُ اللَّهُ قَالَ بَلْ سَمَّانَا اللَّهُ، كُنَّا نَدْخُلُ عَلَى أَنَسٍ فَيُحَدِّثُنَا مَنَاقِبَ الأَنْصَارِ وَمَشَاهِدَهُمْ، وَيُقْبِلُ عَلَىَّ أَوْ عَلَى رَجُلٍ مِنَ الأَزْدِ فَيَقُولُ فَعَلَ قَوْمُكَ يَوْمَ كَذَا وَكَذَا كَذَا وَكَذَا‏.‏