Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "When you see a brother of yours involved in a sin, do not become helpers of Satan against him by saying: 'O Allah, disgrace him! O Allah, curse him!' Rather, ask Allah for well-being for him. For we, the Companions of the Messenger of Allah (peace and blessings be upon him), would not say anything about anyone until we knew upon what his end came. If his end was upon good, we knew that he had attained good, and if his end was upon evil, we feared for him on account of his deeds."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8542