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Ibn Mas'ud reported God's messenger as saying, "You will be helped, granted spoils, and given victory; so, let those of you who attain to that fear God, command what is reputable, and forbid what is disreputable." Abu Dawud transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنِ ابْنِ مَسْعُودٍ عَنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قا ل: «إِنَّكُمْ مَنْصُورُونَ وَمُصِيبُونَ وَمَفْتُوحٌ لَكُمْ فَمَنْ أَدْرَكَ ذَلِكَ مِنْكُمْ فَلْيَتَّقِ اللَّهَ وَلْيَأْمُرْ بِالْمَعْرُوفِ وَلْيَنْهَ عَن الْمُنكر» . رَوَاهُ أَبُو دَاوُد
sunnah.com