Al-Ahad
Allah is Al-Ahad, He is incomparable, unequalled, and indivisible. Al-Ahad is the One who was, is, and will ever remain alone. No one can ever be equal to Him in essence of all His beautiful attributes!
From the root a-h-d which has the following classical Arabic connotations: to be one to be the only one, one alone, sole to unite, unify.
Say, “He is Allah, [who is] One (Quran 112:1)
For your god is one God, so to Him submit. And, [O Muhammad], give good tidings to the humble [before their Lord] (Quran 22:34)
One of the worse things is to make associations with Allah. Here’s passage from Quran regarding this:
And they say, “The Most Merciful has taken [for Himself] a son.” You have done an atrocious thing. The heavens almost rupture therefrom and the earth splits open and the mountains collapse in devastation. That they attribute to the Most Merciful a son. There is no one in the heavens and earth but that he comes to the Most Merciful as a servant. He has enumerated them and counted them a [full] counting. And all of them are coming to Him on the Day of Resurrection alone. Indeed, those who have believed and done righteous deeds – the Most Merciful will appoint for them affection. (Quran 19:88-96)