It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that a man came to the service of the Prophet (peace and blessings be upon him) and said: O Messenger of Allah! How will I know about my good and bad deeds? The Prophet (peace and blessings be upon him) said: "When you hear your neighbors saying about you that you have done a good deed, then indeed you have done a good deed, and when you hear people saying about you that you have done a bad deed, then indeed you have done a bad deed."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 3808