Ibn Qayyim,
, said in al-Wabil al-Sayyib (p.63):
“I heard Shaykul Islaam ibn Taymiyyah,
, say: " Dhikr is to the heart as water is to a fish; see what happens when a fish is taken out of water!
I once attended the morning (fajr) prayer with Shaykul Islaam ibn Taymiyyah, after which he sat down and engaged in dhikr of Allaah until it was almost midday. He then turned to me and said: " This is my morning meal, if I do not partake of it my strength diminishes"
Source: The Exquisite Pearl by Shaykh 'Abdul Rahmaan as-Sa'di,