...Muhammad ibn Ya`qub (al-Kulayni), from `Ali ibn Ibrahim, from his father, from al-Nawfali, from al-Sakuni, from Abu 'Abd Allah (al-'Imam al Sadiq) (A) that he said:
"Amir al-Mu'minin (A) used to say:
`Arouse your heart to contemplation; keep your side clear off the night; and be heedful towards your Lord."
What is the Heart?
There are various applications and denotations of the word `heart'.
For physicians and the common people it is a tiny piece of flesh, whose contractions and expansions cause the flow of blood through arteries and veins, which generates a subtle elan vital.
The philosophers (hukama) use it for a certain seat of the psyche (nafs).
The `urafa' assign to it grades (maratib) and stages (maqamat), and to go into their details is not our concern here.
In the Holy Quran and the ahadith, it has been used both in its general as well as its particular senses in different places.
In the verse `...the hearts reached to the throats ' (33:10), `heart' is used in the same sense as used by physicians.
And in "They have hearts wherewith they understand not, and they have eyes wherewith they see not' (7:179), it is used in the sense used by the philosophers.
And in `Therein verily is a reminder for him who hath a heart, or giveth ear with full intelligence' (50:37), `heart' is used in the same sense as used by the `urafa'.
Imam Khomeini; Forty Hadith, Twenty Seventh Hadith
'O God, carry us in the ships of Thy deliverance, give us to enjoy the pleasure of whispered prayer to Thee, make us drink at the pools of Thy love, let us taste the sweetness of Thy affection and nearness, allow us to struggle in Thee, preoccupy us with obeying Thee, and purify our intentions in devoting works to Thee, for we exist through Thee and belong to Thee, and we have no one to mediate with Thee but Thee!' Imam Sajjad ('A); Sahifa al-Sajjadiyya