Ahadith 2122 – 2123 (below) are repeats. Related posts have been linked (see hyper-linked texts).
Volume 3, Book 43, Number 625:
Narrated Muawiya bin Suwaid:
I heard Al-Bara’ bin ‘Azib (radiallaahu `anhu) saying, “The Prophet (sallallaahu `alayhi wasallam) orders us to do seven things and prohibited us from doing seven other things.” Then Al-Bara’ (radiallaahu `anhu) mentioned the following:-
(1) To pay a visit to the sick (inquiring about his health),
(2) to follow funeral processions,
(3) to say to a sneezer, “May Allah be merciful to you” (if he says, “Praise be to Allah!”),
(4) to return greetings,
(5) to help the oppressed,
(6) to accept invitations,
(7) to help others to fulfill their oaths.
Volume 3, Book 43, Number 626:
Narrated Abu Musa (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) said, “A believer to another believer is like a building whose different parts enforce each other.” The Prophet (sallallaahu `alayhi wasallam) then clasped his hands with the fingers interlaced (while saying that).
Volume 3, Book 43, Number 627:
Narrated Ibn ‘Umar (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) said, “Oppression will be a darkness on the Day of Resurrection.”
Definition: The term “zulm” in Arabic means oppression. The Islamic definition of the word is manifold. They include:
a. transgressing the boundaries on oneself;
b. transgressing the boundaries of others;
c. claiming that which does not belong to oneself;
d. denying others their rights.
“Oppression will turn into darkness on the Day of Resurrection.”
It is known in Islam that the punishment of a person in the Hereafter is similar to what he committed previously in the life of the Dunya. For instance, if a person has two wives and he mistreats one of them, then on the Day of Judgment he will come leaning on one side.
The Prophet (SalAllahu ‘alaihi wassallam) states in these ahadith that oppression will turn into darkness on the Day of Resurrection. The reason for this is when someone oppresses another; he turns the life of another person into darkness. For instance, if one steals someone’s $100 000, he changes that person’s life into difficulties and gloominess. As a retaliation, on the Day of Judgment, Allah will darken the life of the oppressor for what he did.
Meaning of Darkness:
On the Day of Judgment, one will have to cross a bridge in order to gain entry into Paradise. The person’s good deeds will act as a light so that he may cross and bridge and enter into Paradise. If someone oppresses, or is a disbeliever, a hypocrite, or a Muslim with very few good deeds, then they will not receive this necessary brightness to cross the bridge.
Oppression is a Major sin:
In the two hadith that are mentioned above, the Prophet (SalAllahu ‘alaihi wassallam) compares oppression to darkness in the Life-after. This comparison implies that oppression is haram. It is not only haraam but it is worse than haram because it is clear in the sunnah whenever the Prophet (SalAllahu ‘alaihi wassallam) specifies the punishment of an act, that sin is a major sin. If the Prophet (SalAllahu ‘alaihi wassallam) does not mention the punishment of the sin, then it is categorized as a minor one. So this hadith proves that oppression is a major sin.
Taken from: http://ahmadjibril.com/students/thulm.html