:salamext:
So, ascribe not purity to yourselves.) forbidding one from ascribing purity and praising himself and thinking highly of his actions,
[هُوَ أَعْلَمُ بِمَنِ اتَّقَى]
(He knows best him who has Taqwa.) Allah said in another Ayah,
[أَلَمْ تَرَ إِلَى الَّذِينَ يُزَكُّونَ أَنفُسَهُمْ بَلِ اللَّهُ يُزَكِّى مَن يَشَآءُ وَلاَ يُظْلَمُونَ فَتِيلاً ]
(Have you not seen those who claim sanctity for themselves. Nay, but Allah sanctifies whom He wills, and they will not be dealt with unjustly, even equal to the extent of a Fatil.) (4:49) In his Sahih, Muslim recorded that Muhammad bin `Amr bin `Ata said, "I called my daughter, Barrah (the pious one), and Zaynab bint Abu Salamah said to me, `The Messenger of Allah forbade using this name. I was originally called Barrah and he said,
«لَا تُزَكُّوا أَنْفُسَكُمْ، إِنَّ اللهَ أَعْلَمُ بِأَهْلِ الْبِرِّ مِنْكُم»
(Do not ascribe purity to yourselves; Allah knows best who the pious people among you are)' They said, `What should we call her' He said,
«سَمُّوهَا زَيْنَب»
(Call her Zaynab.)'' Imam Ahmad recorded a Hadith from `Abdur-Rahman bin Abi Bakrah, from his father who said, "A man praised another man before the Prophet . The Messenger of Allah said,
«وَيْلَكَ قَطَعْتَ عُنُقَ صَاحِبِكَ مرارًا إِذَا كَانَ أَحَدُكُمْ مَادِحًا صَاحِبَهُ لَا مَحَالَةَ، فَلْيَقُلْ: أَحْسِبُ فُلَانًا وَاللهُ حَسِيبُهُ، وَلَا أُزَكِّي عَلَى اللهِ أَحَدًا، أَحْسِبُهُ كَذَا وَكَذَا، إِنْ كَانَ يَعْلَمُ ذلِك»
(Woe to you, you have cut off the neck of your friend! (He repeated this) If one of you must praise a friend of his, let him say, "I think that so-and-so is this and that; Allah knows best about him and I will never purify anyone before Allah,'' if he knows his friend to be as he is describing him.)'' Al-Bukhari, Muslim, Abu Dawud and Ibn Majah collected this Hadith. Imam Ahmad recorded that Hammam bin Al-Harith said, "A man came before `Uthman bin `Affan and praised him. Al-Miqdad bin Al-Aswad started throwing sand in the face of that man, saying, `The Messenger of Allah ordered us to throw sand in their faces when we see those who praise.'''
Muslim and Abu Dawud also collected this Hadith.
Tafsir Ibn Kathir:
http://www.theholybook.org/en/a.47999.html