Excerpted from the retreat on Tazkiyah tun Nafs by HY May 2011
رَبَّنَا وَٱبۡعَثۡ فِيهِمۡ رَسُولاً۬ مِّنۡہُمۡ يَتۡلُواْ عَلَيۡہِمۡ ءَايَـٰتِكَ وَيُعَلِّمُهُمُ ٱلۡكِتَـٰبَ وَٱلۡحِكۡمَةَ وَيُزَكِّيہِمۡۚ إِنَّكَ أَنتَ ٱلۡعَزِيزُ ٱلۡحَكِيمُ (١٢٩)
“Our Lord! send amongst them a Messenger of their own who shall rehearse Thy Signs to them and instruct them in Scripture and Wisdom, and purify them; for Thou art the Exalted in Might the Wise.” (129)
Within the body of this supplication of Ibrahim AS are three supplications asking Allah Subhanawataala to Send a Messenger from their own folks who will:
1. Rehearse or recite the tilawah of the word of Allah
2. He will teach the word of Allah (The Quran and Sunnah)
3. And by doing they will be purified (Tazkiyah
He is the Mighty) Aziz ul Hakeem and uses his Wisdom to purify His ebad
FULFILLMENT OF THE ABOVE DUA 2:151
كَمَآ أَرۡسَلۡنَا فِيڪُمۡ رَسُولاً۬ مِّنڪُمۡ يَتۡلُواْ عَلَيۡكُمۡ ءَايَـٰتِنَا وَيُزَكِّيڪُمۡ وَيُعَلِّمُڪُمُ ٱلۡكِتَـٰبَ وَٱلۡحِڪۡمَةَ وَيُعَلِّمُكُم مَّا لَمۡ تَكُونُواْ تَعۡلَمُونَ (١٥١)
A similar (favor have ye already received) in that We have sent among you a Messenger of your own rehearsing to you Our signs, and purifying you, and instructing you in Scripture and wisdom, and in new Knowledge. (151)
Thus Allah Subhanawataala says that I have fulfilled the promise to Ibrahim (AS), I have provided the messenger (Prophet Muhammad pbuh), the book (The Quran) and the methodology (Sunnah) to practice it in order to purify you.
Thus the Ulema (Scholars extrapolate from this that Tazkiyah (Purification of the Soul) is a fard or obligatory.