Conceal Faults of Your Fellow Muslim
Imam Baqir (a.s) said: 'It is compulsory for every Muslim to conceal even seventy great faults of his fellow Muslims (in order to save his prestige).' (His faults should be dealt with personally but not be spread among the members of society).
Bihar-ul-Anwar, vol. 74, p. 301
Source: al-shia.org
Other links:
Righteousness of Intentions
Quran, a source of advice
Control your temper
True worship
Fatal Vices
Laziness
Piety