Laqad sadaqa Allahu rasoolahu arru'ya bilhaqqi latadkhulunna almasjida alharama in shaa Allahu (surah Al Fath: 27)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(48:27) Allah indeed showed His Messenger the true vision, *47 one fully in accord with reality. If Allah so wills *48 you shall certainly enter the Inviolable Mosque, in full security, *49 you will shave your heads and cut your hair short, *50 and do so without any fear. He knew what you did not know, and He granted you a victory near at hand even before (the fulfilment of the vision).

Laqad sadaqal laahu Rasoolahur ru`yaa bilhaqq, latadkhulunnal Masjidal-Haraama in shaaa`al laahu aamineena muhalliqeena ru`oosakum wa muqassireena laa takhaafoona fa`alima maa lam ta`lamoo faja`ala min dooni zaalika fathan qareebaa

*47) This is the answer to the question that was constantly agitating the minds of the Muslim. They said, "The Holy Prophet had seen in his vision that he had entered the Masjid al-Haram and moved round the Ka`bah in worship. Then how is it that they were returning without performing `Umrah?" In answer to this, although the Holy Prophet had told them that in his vision he had not seen that they would perform the `Umrah that very year, still there remained some suspicion in the hearts. Therefore, Allah Himself explained that it was He Who had shown the vision and it was a true vision and it would certainly be fulfilled.
*48) Here, about the words insha'-Allah (if Allah so wills), which Allah Himself has used with His promise, one can raise the objection that when Allah Himself is making this promise, what is the meaning of making it conditional upon His own willing it? The answer is: Here the words insha'-Allah have not been used in the sense that if AIlah dces not will, He will not fulfil His promise, but in fact these relate to the background in which this promise was made. The presumption on the basis of which the disbelievers of Makkah had played the drama of preventing the Muslims from `Umrah was that only he whom they would allow would perform `Umrah, and would perform it only when they would allow it At this AIIah has said: "This depends on Our, not on their, will. The reason why 'Umrah has not been performed this year is not because the disbelievers of Makkah did not allow it to be performed, but because We did not will it to be performed; in the future 'Umrah will be performed if We will, no matter whether the disbelievers allow it or disallow it." Besides, these words also contain the meaning that the Muslims too, will perform `Umrah not by their own power but because We would will that they should perform it; otherwise if We do not will, they do not possess any power to perform it by themselves."
*49) This promise was fulfilled in the following year in Dhil-Qa'dah A. H. 7. This `Umrah is well known in history as 'Umrah al-Qada `.
*50) The words clearly point out that it is not obligatory to get the head shaved in `Umrah and Hajj, but it is also right to get the hair cut short. However, it is better to have the head shaved, for Allah has mentioned it first and then mentioned having the hair cut short.