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

الرَّوْحُ والرّاحَةُ في الرِّضا واليَقينِ، والهَمُّ والحَزَنُ في الشَكِّ والسَّخَطِ.

'Tranquility and comfort lie in satisfaction and certainty, whereas worry and sorrow lie in doubt and dissatisfaction.'

Source: Mishkat al-Anwar, no. 34