Hawzah News Agency – Imam al-Sadiq peace be upon him says:

اِحذَروا أهواءكُم كما تَحذَرونَ أعداءكُم، فلَيسَ شَي‏ءٌ أعدى‏ لِلرِّجالِ مِن اتِّباعِ أهوائهِم، وحَصائدِ ألسِنَتِهِم .

'Be on your guard against your desires as you guard against your enemies, for there is nothing worse as an enemy to men than following their desires and the consequences of their tongues [i.e. speech].'

Source: al-Kafi, v. 2, p. 335, no. 1