It is narrated from the third Shia Imam, Hussain ibn Ali, Seyyedu-Shuhada (AS) who said:

Swear to my life, Imam is the one who rules according to the Book of Allah (SWT); rises up for justice; is devoted to the rightful religion, and pledges his life in the way of Allah (SWT) [1]. Imam Hussain (AS) and his true followers like his representative of the time Moslim ibn Aqeel (AS) emphasized these characteristics under any circumstances. When Moslim (AS) was captured and faced Yazid’s agent Obaidullah bin Ziad in Governor’s house in Kufa, pointed to this great aim of Imam’s (AS) uprising in order to establish justice. Ibn Ziad asked Moslim (AS): For what purpose you have come to Kufa and invite the people to revolution? Moslim (AS) replied: We have come to command justice and invite to the Rule of Allah (SWT) [2].

[1] Kitabul Qaibah, Nu’amaani, Page 206

[2] Tareekh Tabari, Vol. 4, Page 22.

Source: en.shafaqna.com