Tazkiyyah means purification and tazkiyyah an-nafs is the purification of the self (soul). This is a fundamental part of Islam and the servant's relationship with his Lord, Allah (سبحانه وتعالى). Whilst one is busy in the deep study of Fiqh, Hadith, and other Islamic sciences, this aspect of Islam can be neglected when it is in fact the most important of them all in terms of attaining khushoo and sweetness in Salah, in improving one's character and developing noble qualities such as modesty, humbleness and gheerah (protective jealousy).
The best and first cure for hardheartedness is to obey the commands of Allah and to refrain from that which Allah has prohibited. This is the essence of the word Islam, and the muslim is the one who submits himself to Allah. It is incumbent upon the muslim to learn what is obligatory for him to do and what it obligatory for him to refrain from, ignorance is not an excuse for those who have the ability to learn and seek knowledge, just like one seeks food and drink to nourish his body, it is similiarly incumbent upon him to seek the obligatory knowledge that will nourish his soul. Especially so in the modern era where volumes of books that used to fill entire libraries can be downloaded into devices which can fit into the palm of one's hand. If one remains steadfast on this then he will, by the mercy of Allah, be admitted to Jannah insha'Allah.
However, there are others who are blessed by Allah with special foresight, the ambitious ones, the ones who aren't satisfied with the minimum, they are the reciters and the memorisers of the Quran, they are the one's who strive with their lives and their wealth in the cause of Allah, they are the patient when the calamities strike, they are the one's who stand the night while everyone else is sleeping, they are one's who fast on alternate days, they love for the sake of Allah and they hate for the sake of Allah, they give for the sake of Allah and they hold back for the sake of Allah, they are the one's whose fear of Allah increases at times of seclusion and they are the one's who confront falsehood no matter the consequences. These are the one's who will attain Firdaws by the will of Allah.
It is as the Messenger of Allah (صلى الله عليه وسلم) said in a hadith qudsi:
“Allah says: ‘Whoever takes a close friend of Mine as an enemy, I declare war on him. My slave does not draw closer to Me by anything more beloved to Me than that which I have made obligatory upon him, and My slave continues to draw closer to Me by doing naafil (supererogatory) deeds until I love him, and if I love him I will be his hearing with which he hears, his vision with which he sees, his hand with which he strikes and his foot with which he walks. If he were to ask of Me, I would surely give to him; if he were to seek refuge with Me, I would surely grant him refuge. I do not hesitate about anything that I want to do as I hesitate to take the soul of a believer, for he hates death and I hate to hurt him.”
Narrated by al-Bukhaari (6502).