"Who is a true believer?" Ahadith of Prophet Muhammed (s)

 لَيْسَ المُؤْمِنُ بِالطَّعَّانِ وَلا اللَّعَّانِ وَلا الفَاحِشِ وَلا البَذِي
A (true) believer does not taunt, damn, slander and abuse people.

لا تَجْتَمِعُ خَصْلَتانِ في مُؤْمِنٍ: البُخْلُ وَالكِذْبُ
A (true) believer is bereft of two attributes: telling lie and stinginess.

يَنْبَغِي أنْ يَكُونَ لِلْمُؤْمِنِ ثَمانِيَةُ خِصالٍ: وَقارٌ عِنْدَ الهَزائِزِ؛ وَصَبْرٌ عِنْدَ البَلاءِ؛ وَشُكْرٌ عِنْدَ الرَّخاءِ؛ وَقُنُوعٌ بِما رَزَقَهُ اللَّهُ عَزَّ وَجَلّ؛ لا يَظْلِمُ الأَعْداءَ؛ وَلا يَتَحامَلُ عَلَى الأصْدِقاءِ؛ بَدَنُهُ في تَعَبٍ وَالنّاسُ مِنْهُ في راحَةٍ.
A (true) believer should have the following eight characteristics: soberness at hard times, patience in calamities, gratefulness in reliefs, contentment for whatever the Great and Glorified God has given, avoidance from oppressing enemies and hurting friends, and toleration of physical sufferings to make people feel safe from him.

المُسْلِمُ مَنْ سَلِمَ المُسْلِمُوْنُ مِنْ لِسَانِهِ وَيَدِهِ
A (true) Muslim is the one from whose tongue and hand, Muslims at rest stand.

تَجِدُ المُؤْمِنُ مُجْتَهِدَاً فِيمَا يُطِيْقُ مُتَلَهِّفَاً عَلى ما لا يُطِيْقُ
A believer does his best in what his ability demands, but craves for what beyond his power stands.

(From the book Nahjul-Fasah, peak of rhetoric)