It was narrated that Abu Hurayrah said: I heard the Prophet (ﷺ) say: “Whoever sees me in a dream will see me when he is awake; the Shaytaan cannot take my shape.” (Narrated by al-Bukhaari, 6592; Muslim, 2266)

Narrated Anas: The Prophet said, “Whoever has seen me in a dream, then no doubt, he has seen me, for Satan cannot imitate my shape. [Sahih Bukhari -Book #87, Hadith #123]

Narrated Abu Huraira: I heard the Prophet saying, “Whoever sees me in a dream will see me in his wakefulness, and Satan cannot imitate me in shape.”

Abu ‘Abdullah said, “Ibn Sirin said, ‘Only if he sees the Prophet in his (real) shape.’” [Sahih Bukhari - Book #87, Hadith #122]

What does this mean: "...will see me when he is awake."

Also what does "..(real) shape mean."

Does it imply that a person who sees the Prophet (ﷺ) in a dream must already know what he looks like from hadith? So an illiterate person, or someone who has not studied the shamaail cannot see the Prophet (ﷺ) in a dream?

Firstly we thank Allah, the One who is ever living, the only One worthy of worship who sent us His beloved Prophet (ﷺ), a man who lived among men who then died after finishing his mission.

I will try to break down the answer into different points:

1) What does this mean: "...will see me when he is awake."

They are few explanations given by the scholars but I will just mention a few mentioned by Imaam Nawawi:

1. It applies to the time when the Prophet (ﷺ) was still alive. Those people who had not seen the Prophet (ﷺ) in real life and were not residents of Medinah, when they saw the Prophet (ﷺ) in their dreams they would go on to see him in real life
2. It means that this is bushrah/glad tidings for a believer because he will be in the company of the Prophet (ﷺ) on the day of Qiyamah[1]

2) What does "..(real) shape” mean?

It means the qualities he sees in the dream must be the same qualities which are related to us in the authentic narrations. If the narration for example says he had long hair and then a person sees someone with short hair saying he is the Prophet (ﷺ) then he has not seen the Prophet (ﷺ). Because of this the salafus salih used to ask for the description of the Prophet (ﷺ) from whoever claimed to have seen Prophet Muhammad.[2]

3) Does it imply that a person who sees the Prophet (ﷺ) in a dream must already know what he looks like from hadith?

I don’t think it implies that, as there are chances that can happen without the person having understanding of the hadith, but is just a true Muslim and a true believer.

If a man told Ibn Sireen that he had seen the Prophet (ﷺ) [in a dream], he would say, “Describe to me the one whom you saw.” If he gave a description that he did not recognize, he would say, “You did not see him.”[3]

From the above we can say the ones who used to go to Ibn Sireen then relate to him about their dreams didn’t know much about the description of the Prophet (ﷺ) and thus they went to ask the scholar

4) So an illiterate person, or someone who has not studied the shamaail cannot see the Prophet (ﷺ) in a dream?

It will be wrong to say that as partly explained above; also being illiterate is not a sign that a person does not know the Prophet (ﷺ). Some people (most of them who are way better believers than any of us) could not read nor write, yet they knew the Prophet (ﷺ).

Another important thing to note is:

Those who claim to see the Prophet (ﷺ) while awake are liars and they go against the teachings of Islam:

The scholars of the Standing Committee said, refuting the beliefs of al-Tijani:

“There is no proven report from the Rightly-Guided Caliphs or from the rest of the sahabah (may Allah be pleased with them), who are the best of mankind after the prophets, that any of them claimed to have seen the Prophet (peace and blessings of Allah be upon him) when he was awake.” [4]

To conclude: It is very important to know that believing in the Prophet (ﷺ) and following the path trodden by him by his Sahabaah is what is really important. One has to do whatever they can to emulate this great man as that will make Allah love us. Allah says in the Qur’an:

“Say: If you love Allah, then follow me, Allah will love you and forgive you your faults, and Allah is Forgiving, Merciful”[5]

May Allah grant us jannatul firdaus and may he make us rise on the day of judgment in the company of His beloved Prophet Muhammad (ﷺ).

Only Allah knows best.

References:

[1] Sahih Muslim: 2266, Sharh, Imaam Nawawi

[2] http://islamqa.com/en/ref/14052/seeing%20the%20prophet%20in%20a%20dream

[3] Fath al-Baari, 12/383, 384

[4] http://islamqa.info/en/cat/2023

[5] Surah Ale-Imran, 3:31