it is mentioned in scriptures!!! sikhi does speak about muhammad coming to the earth!!!! By gurdas ji's vaars where described by Guru Arjun dev ji as being the key to Guru Granth sahib ji. here is what it says in on of them. read the following what bhai Gurdas ji says:
When varied sects got prevalent, then Muhammad, the beloved of God was born.
The nation got divided into seventy two divisions and many types of enmity and opposition erupted.
The world was bound to roza, id, namaz, etc.
Pirs, paigambars aulias, gaus and qutabs came into being in many countries.
The temples were replaced by mosques.
Less powerful were killed and thus the earth became replete with sin.
Armenians and Rumis were declared apostates (Kafirs) and they were decimated in the Battle fields.
The sin became ubiquitious all around.
There are four castes of Hindus and four sects of Muslims in the world.
The members of both religions are selfish, jealous proud, bigoted and violent.
The Hindus make pilgrimage to Hardvar and Banaras, the Muslim to the Kaba of Mecca.
Circumcision is dear to the Muslims, sandal mark (tilak) and sacred thread to the Hindus.
The Hindus invoke Ram, the Muslims, Rahim, but in reality there is only One God.
Since they have forgotten the Vedas and the Katebas, worldly greed and devil have led them astray.
Truth hidden from both; the brahmins and maulvis kill one another by their animosities.
Neither sect shall find liberation from transmigration.
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The benefactor Lord listened to the cries (of humanity) and sent Guru Nanak to this world.
......He preached in this darkage (kaliyug) that, saragun (Brahm) and nirgun (Parbrahm) are the same and identical.
Dharma was now established on its four feet and all the four castes (through fraternal feeling) were converted into one caste (of humanity).
.........Inverse is the game of the beloved; he got the egotist high heads bowed to feet of the lord.
Baba Nanak rescued this dark age (kaliyug) and recited ‘satinam’ (true name) mantr for one and all.
Guru Nanak came to redeem the kaliyug (age of darkness).
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First of all Baba Nanak obtained the gate of the grace (of Lord) and then He underwent and earned the rigorous discipline( of heart and mind).
He fed himself with sand and swallow-wort and made stones his bedding i.e. he enjoyed poverty too.
He offered his full devotion and then he was fortunate to have proximity with God.
Baba reached the region of truth wherefrom he received Nam, the storehouse of nine treasures and humility.
In his meditation, Baba found the whole earth burning (with the fire of lust and anger).
Without the taker from darkness to light there is utter darkness and he heard the cries of the common men.
To further understand the people, Guru Nanak donned robes in their manner and preached them to be detached (from the pleasure and pain).
Thus he went out to depurate humanity on earth.
