After the War of Jamal, Imam Ali (AS) wrote a letter to Ash’ath ibn Qais:

Indeed, official post and governance is not a mean of attaining bread and water (wealth), rather it is a trust around your neck (you have been entrusted with this official post). You must obey your commander and Imam; you have no right to oppress people, nor are you allowed to act upon an important task without having an official order [1].

[1] Nahjul Balaghah, Letter 5.

Source: en.shafaqna.com