Sahih Muslim
The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wal-Adab)
Book Number: 032 / Hadith Number: 6250
Salim reported on the authority of his father that Allah's Messenger (may peace be upon him) said: 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.