Sahih Muslim Book 45, Hadith 75
Oppression is the darkness on the Day of Resurrection.
Be on your guard against committing oppression, for oppression is a darkness on the Day of Resurrection, and be on your guard against stinginess for stinginess destroyed those who were before you, as it incited them to shed blood and make lawful what was unlawful for them.
A Muslim is the brother of a fellow-Muslim. He should neither commit oppression upon him nor ruin him, and he who meets the need of a brot'ier, Allah would meet big needs, and he who relieved a Muslim from hardship Allah would relieve him from the hardships to which he would be put on the Day of Resurrection, and he who did not expose (the follies of a Muslim) Allah would conceal his follies on the Day of Resurrection.