Abul Husayn al-Nuri (circa 840 - 907 CE) was one of the earliest Sufis who wrote on the subject of qalb (heart) in Maqamat al-Qulub (Stations of the Hearts). This great sufi of Baghdad was known as Amir al-qulub (Prince of Hearts) whose language Attar described as latif zarif (fine and elegant).
So know, O son, that your heart has four levels:
1. Breast/chest 2. Heart 3. Inner heart 4. Innermost kernel of the heart.
In Arabic, they are known as sadr, qalb, fu'ad and lubb.
According to al-Nuri, sadr corresponds to Islam; qalb to iman; fu'ad to ma'rifa and lubb to tawhid. All of these are based on the Quranic verses:
SADR: Is he whose breast God has opened to Islam. Quran 39:22
QALB: But God has endeared faith to you, adorning your hearts with it. Quran 49:7
FU'AD: The heart lied not (in seeing) what it saw. Quran 53:11
LUBB: There are signs for people of pith/wisdom/understanding. Quran 3:190
In this last verse referred, it is understood that the root word for albab (people of pith/wisdom/understanding) is in fact lubb which al-Nuri relates to tawhid. Lubb being the innermost kernel of the heart is where tawhid resides.
O son, take heed, this insights to the layers of our hearts came from an extraordinary man who acquired his surname (al-Nuri) because he radiated light (nur) when talking! Allahu A'lam bissawab!
Source: Essay by Annemarie Schimmel in The Heritage of Sufism