The hadith you are asking is about is in Sahih al-Bukhari:
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا مُعْتَمِرٌ، عَنْ حُمَيْدٍ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم: انْصُرْ أَخَاكَ ظَالِمًا أَوْ مَظْلُومًا. قَالُوا يَا رَسُولَ اللَّهِ هَذَا نَنْصُرُهُ مَظْلُومًا، فَكَيْفَ نَنْصُرُهُ ظَالِمًا قَالَ: تَأْخُذُ فَوْقَ يَدَيْهِ
Narrated Anas: Allah's Messenger (ﷺ) said, "Help your brother, whether he is an oppressor or he is an oppressed one. People asked, "O Allah's Messenger (ﷺ)! It is all right to help him if he is oppressed, but how should we help him if he is an oppressor?" The Prophet (ﷺ) said, "By preventing him from oppressing others."
— Sahih al-Bukhari, Book 46, Hadith 5
It can also be found in Sahih Muslim 45/80, Jami' at-Tirmidhi 33/98, Sahih al-Bukhari 89/13 with slightly different wording, among other books of hadith.