When the Ansar performed the Pilgrimage, they did not enter their houses but from behind. A person from the Ansar came and he began to enter from his door but it was said to him (why he was doing something in contravention to the common practice of coming to the houses from behind). Then this verse was revealed." Piety is not that you come to the doors from behind" (ii. 189).  

سَمِعْتُ الْبَرَاءَ يَقُولُ كَانَتِ الأَنْصَارُ إِذَا حَجُّوا فَرَجَعُوا لَمْ يَدْخُلُوا الْبُيُوتَ إِلاَّ مِنْ ظُهُورِهَا قَالَ فَجَاءَ رَجُلٌ مِنَ الأَنْصَارِ فَدَخَلَ مِنْ بَابِهِ فَقِيلَ لَهُ فِي ذَلِكَ فَنَزَلَتْ هَذِهِ الآيَةُ {وَلَيْسَ الْبِرُّ بِأَنْ تَأْتُوا الْبُيُوتَ مِنْ ظُهُورِهَا}  

I heard Al-Bara' saying, "The above Verse was revealed regarding us, for the Ansar on returning from Hajj never entered their houses through the proper doors but from behind. One of the Ansar came and entered through the door and he was taunted for it. Therefore, the following was revealed: -- "It is not righteousness That you enter the houses from the back, But the righteous man is He who fears Allah, Obeys His order and keeps away from What He has forbidden So, enter houses through the proper doors." (2.189) (Using translation from Bukhārī 1803)   

سَمِعْتُ الْبَرَاءَ يَقُولُ كَانَتِ الْأَنْصَارُ إِذَا حَجُّوا فَجَاءُوا لَا يَدْخُلُونَ مِنْ أَبْوَابِ بُيُوتِهِمْ وَلَكِنْ مِنْ ظُهُورِهَا فَجَاءَ رَجُلٌ مِنَ الْأَنْصَارِ فَدَخَلَ مِنْ قِبَلِ بَابِهِ فَكَأَنَّهُ عُيِّرَ بِذَلِكَ فَنَزَلَتْ هَذِهِ الْآيَةُ {وَلَيْسَ الْبِرُّ بِأَنْ تَأْتُوا الْبُيُوتَ مِنْ ظُهُورِهَا وَلَكِنَّ الْبِرَّ مَنِ اتَّقَى وَأْتُوا الْبُيُوتَ مِنْ أَبْوَابِهَا} [البقرة 189]