Praise be to Allah,
Hushaym narrated to us from ‘Ubaydullaah ibn Abee Bakr from Anas [ibn Maalik] and Yoonus [ibn ‘Ubayd ibn Deenaar] from al-Hasan who said, “Allah’s Messenger (PBUH) said,
‘Help your brother whether he is an oppressor or he is oppressed.’ Someone· asked, ‘Messenger of Allah, it is all right to help him if he is oppressed, but how can I help him when he is an oppressor?’ He replied, ‘You help him by preventing him from oppressing others.'”
(Musnad Ab.mad, 3/99. The first isnaad of this Hadeeth is Saheeh according to the conditions set by al-Bukhaaree and Muslim, but the second one is mursal.)
Source:
Thulaathiyyaat Musnad Ahmad ibn Hanbal (may Allah have mercy on him)
Reports of Imaam Ahmad that have only three narrators between him and the Prophet, peace, and blessings be upon him.
Shaykh Muhammad ibn Ahmad ibn Saalim as–Saffaareenee al–Hanbalee ~(1114 –1188 AH)