"One day while the Prophet was sitting in the company of some people, (the Angel) Gabriel came and asked, "What is Faith?" Allah's apostle replied, "Faith is to believe in Allah, his angels, (the) meeting with him, his apostles, and to believe in The Resurrection." Then he further asked, "What is Islam?" Allah's apostle replied, "To worship Allah alone and none else, to offer prayers perfectly, to pay the compulsory charity (Zakat) and to observe fasts during the month of Ramadan." Then he further asked, "What is Ihsan?" Allah's apostle replied, "To worship Allah as if you see him, and if you cannot achieve this stage of devotion then you must consider that he is looking at you.

There are three major parts and parcels of the teaching of Islam, the teaching of Qur'an and Sunnah. In the above tradition of the Holy prophet (Peace Be Upon Him) reported by his companion Hadrat Abu Hurrairah, He reported that one day the Holy Prophet was sitting with his companions and (the angel) Gabriel came to him. He sat near to the Prophet and asked, "What is Islam?" This was the first question. The Holy Prophet replied that Iman is to believe in Allah, his angels, the meeting with him, his apostles, to believe in Resurrection and in His books. So the Holy Prophet prescribed the faith as the reply of the question of Iman

Then he asked the second question, "What is Islam?"
The Holy Prophet Replied, "To worship Allah without any Participation, to offer prayers perfectly and to pay compulsory Charity (Zakat) and you should observe the fast during the month of Ramadan and the performance of Haj as well."

Then he asked the third question," What is Ihsan?"
The Holy Prophet did not say that Din is complete by Iman and Islam. Had been there no third stage then Gabriel would have never asked the third question. He asked the first question about Iman, second about Islam and third about Ihsan.

Now the question is "What is Ihsan?" Cardinal

The Holy Prophet replied that Ihsan is Cardial and Spiritual condition. How that you should worship Allah in such a condition as if you see him, and if you cannot see him, then at least you should achieve this condition that he is seeing you.

So here two conditions have been mentioned:
First that you see him, this is the condition of observation.
Second that he is seeing you, this is the condition of meditation.

The first thing establishes from the tradition of the Holy Prophet Mohammed (Peace Be Upon Him) that there are three stages Iman, Islam and Ihsan. Iman is faith, Islam is Practice and Ihsan is Spirituality.

So according to Muslim scholars and leaders, this concept of Ihsan became the basis of Tasawuf (Sufism), became the basis of spiritualism, became the basis of promotion of spirituality in human personality because this would be the cardial condition, a spiritual attachment of the man with his Lord, Almighty God. That's why, same is the aspect which has been mentioned as one of the primary duties of the Holy Prophet and Tazkkiyatun Nafs (The purification of the mind and soul). It is stated in the Holy Qur'an:

"Allah did a great favour on the believers that he sent a prophet among themselves, reciting onto them the verses (of the Holy Qur'an), purifying them and teaching in Book and wisdom……."

There are four prophetic duties of Prophet Mohammed (Peace Be Upon Him) which are mentioned in the Qur'anic verse.

a. The recitation of The Qur'an on the companions (Yatlu Alaikum Ayaatuna)

b. Purification of mind and soul (Fayuzakkeekum)
This is the internal Purification. This is the basis of Tasawwuf, the basis of Tareeqa, the basis of Saluk, and the basis of spiritual purification of man.

c. The Knowledge of Book

d. The knowledge of Wisdom

This concept has been given by the Holy Qur'an. It has been elaborated at many occasions. For example, in The Holy Qur'an it is stated:

"Only those will prosper who will purify themselves"

Surely, only that man is succeeded who has purified the mind and after purification he had offered his prayers. He has reminded the name of Almighty Allah and after this remembrance he has offered his prayers.

So purification of mind and constant remembrance of Almighty Allah and offering of prayer, if we combine these things, the combination of these teachings becomes Tasawwuf, this becomes Hus-ne-Ikhlas.

It is said by Imam Malik binUns:

"If man gets the light from Fiqh-ul-Islam (The apparent teachings of Islam) and leaves the tasawwuf (Spiritual teaching of Islam) he attaches himself with Fisq (impiety). And if man attaches himself with Tasawwuf and detaches himself from Fiqh-ul-Islam, he becomes an atheist. And only that man reaches the destination and gains the Haq (Truth) who combines the apparent teaching of Islam as well as the internal aspect of teaching of Islam."

Externality is Fiqh and internality of Deen is Tasawwuf. We can understand this thing through this prophetic tradition which is also reported by Imam Hasan Basri. He states:

"There are two kinds of knowledge. First is the Knowledge of heart and second is the knowledge of tongue. The knowledge of heart is really the useful knowledge and the second is the knowledge of tongue, this has been made mandatory and compulsory for the mankind to be followed"

So Tasawwuf is nothing conflicting with deen. This is nothing outside the deen. To understand what is Tasawwuf and its place in deen, this can be understood through this example that if we offer our prayer, we make Wudu (Abolution). We offer Salah while sitting towards the direction of Ka'abaa. So as far as Fiqh-ul-Islam is concerned, if our face is towards the Ka'abaa our Salah is perfect. As far as Tasawwuf is concerned, if our Kalb (Heart) is towards Rab-ul-Ka'abaa (The Almighty God), then our Salah is perfect

As we purified our body in the form of Gusl (Bath), Wudu (Ablution), Tasawwuf requires the purification of the heart from every kind of sin so that man gets purification and enlightenment of the soul.

So this is the concept of Ihsan given by hadith and the Qur'an. In Qur'an it is Tazkiya, in the Hadith it is Ihsan and in Islamic terminology, it is known as Tariqa and Tasawwuf.