Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- Muslims believe that the ultimate source of the knowledge of the unseen realities is Allah and Allah alone.
2- Allah may disclose matters of the unseen to whomever He wills among His Messengers.
In responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
The matters of the Unseen (ghayb) are indeed the exclusive realm of Allah the Almighty; and in so far as anyone else has been given access to something of it, it is an exception.
Allah the Almighty orders the Prophet Muhammad (peace be upon him) to declare that He is a mortal like everyone else and that he does not know the unseen realities. We read in the Quran:
{Say, [O Muhammad], “I do not tell you that I have the depositories [containing the provision] of Allah or that I know the unseen, nor do I tell you that I am an angel. I only follow what is revealed to me.” Say, “Is the blind equivalent to the seeing? Then will you not give thought?”} (Al-Anam 6:50)
However, the verse does not contradict the fact that sometimes Allah chooses to disclose something of the hidden realities to His chosen messengers from among angels and humans: This is what we learn from the following verses:
{[He is] Knower of the unseen, and He does not disclose His [knowledge of the] unseen to anyone. Except whom He has approved of messengers, and indeed, He sends before each messenger and behind him observers.} (Al-Jinn 72: 26-27)
Therefore, there is no contradiction. While the Unseen realities are the exclusive realms of Allah, He may choose to disclose some knowledge of it to some of his chosen messengers.
Therefore, we need not be surprised if we learn that Allah allowed His Messenger to see what was happening behind him in Prayer.
In other words, they are part of this special favors bestowed on him by Allah.
Abu Hurairah reported that the Prophet (PBUH) said:
“Do you consider or see that my face is towards the Qibla? By Allah, neither your submissiveness nor your bowing is hidden from me, surely I see you from my back.” (Al-Bukhari)
The Quran tells us that the Prophet Ibrahim (peace be upon him) entered the blazing fire-pit and emerged out of it unscathed and unharmed. We read in the Quran:
{Allah said, “O fire, be coolness and safety upon Abraham.”} (Al-Anbiya 21:69)
The Prophet Moses parted the red sea and hit the ground with his staff transformed into a serpent. We read in the Quran:
{Then We inspired to Moses, “Strike with your staff the sea,” and it parted, and each portion was like a great towering mountain.} (Ash-Shu`ara 26:63)
Prophet Jesus (peace be upon him) was given the power to revive the dead, heal the leper and restore the eyesight to the blind. We read in the Quran:
{And [make him] a messenger to the Children of Israel, [who will say], ‘Indeed I have come to you with a sign from your Lord in that I design for you from clay [that which is] like the form of a bird, then I breathe into it and it becomes a bird by permission of Allah . And I cure the blind and the leper, and I give life to the dead – by permission of Allah . And I inform you of what you eat and what you store in your houses. Indeed in that is a sign for you, if you are believers.} (Aal Imran 3:49)
Likewise, the Prophet Muhammad (peace be upon him) was saved miraculously from his enemies’ many attempts to assassinate him.
However, nothing of this fact would contradict the principle we are to hold firmly to that the ultimate source of the knowledge of the unseen realities is Allah and Allah alone.
Almighty Allah knows best.