For dhikr see this excellent article:
God, the Exalted, said: "Verily in the remembrance of God (dhikr Allah) do hearts find rest!" (13:28). If it is asked: How is the meaning of this verse reconciled with that of His saying: "They only are the true believers whose hearts feel fear (wajilat = tremble or shake) when God is mentioned (dhukir Allah)" (8:2) [ more ]