It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "O people! Beware of oppression, for oppression will be darknesses on the Day of Resurrection."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5662
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد حسن .