The belief in the Existence of Allaah
It is the full attestation that Allaah, The One free of all imperfection, The Most High, exists. No one openly denied the existence of Allaah, The Mighty and Majestic, except by way of haughtiness. Otherwise it is impossible for any reasonable person to claim that this creation was appeared by chance or it came into existence without a cause, due to the fact that this is something impossible to occur as agreed upon by the people of sound intellect.
Indeed the existence of Allaah, The Mighty and Majestic, is something proven by all kinds of evidences:
(ii) Fitriyyah [from Fitrah: Natural disposition],
(iii) Legal, and
(iv) Hissiyyah [from Hiss: What is experienced and perceived].
All of these four evidences prove the existence of Allaah, The Mighty and Majestic.
As for the intellectual proof, then it is that we witness the existence of this universe and what occurs in it from things which is rather impossible for any creature to do. The existence of creation: the heavens, the earth and what is in them from the stars, mountains, rivers, trees, the speaking, the speechless, and so forth. How did this existence come into being? Did it appear by chance? Or did it occur without a cause? Or did it bring itself into existence? These are three possibilities whereby the intellect does not accept a fourth possibility. All of these possibilities are futile and false.
As for it appearing by chance, then this is something that the intellect as well as reality itself denies, because you cannot bring such magnificent creation into existence just by chance; there must be a cause for each effect. Furthermore its astounding arrangement and harmonious order that has no conflict or collision within it, makes it impossible that it all came about randomly. This is because that which would come into existence by chance, its development would not be ordered because all of it is random and by chance.
As for the possibility that creation brought itself into existence, then it is also clearly impossible. This is because before its existence, creation did not exist, it was nothing; and that which is nothing cannot bring into existence that which does not exist.
As for the third possibility that this existence came to exist without a cause, then the meaning is the same as our saying that it appeared by chance; and this, as preceded, is impossible.
It remains to be said that existence has someone who brought it into being, and He is Allaah, The Mighty and Majestic. He, The Most High, Says:
Were they created by nothing? Or were they themselves the creators? (did they create themselves?) Or did they create the heavens and the earth? Nay, they have no certainty? [Qur’aan, soorat at-Toor (52): 35-36].
Hence, this creation rationally proves the existence of Allaah, The Mighty and Majestic.
As for the proof of the natural disposition for the existence of Allaah, then it is so evident that it does not require a proof. This is because man is created upon the natural disposition (Fitrah) of belief in his Rabb. The Prophet (Sallallaahu alaihi wa sallam) said:
“Every child is born upon the natural disposition, then his parents change him into a Jew, or a Christian, or a Majian.” 
That is why in case something suddenly befalls upon a person in this life such that it may destroy him, he would say with his tongue and without feeling: “O Allaah!” or “O Rabb!” or the like. This proves that the innate nature of man has been created upon the belief in the existence of Allaah, The Mighty and Majestic.
As for the proof of the Hiss (i.e What is experienced and perceived) for the existence of Allaah, then it is very often that we hear of supplications being answered by Allaah. Also the person himself who calls upon Allaah, his supplications are indeed responded to. Many are those who invoked Allaah and said: “O [Yaa] Rabb,” and behold! They saw response was right before his eyes! Indeed in the Qur’aan there are many examples for this, like the Saying of Allaah, The Most High:
And (remember) Ayyub (Job), when he cried to his Rabb: “Verily, distress has seized me, and you are the Most Merciful of all those who show mercy”. So We answered his call, and We removed the distress that was on him. [Qur’aan, soorat al-Anbiyaa’ (21): 83-84].
In addition, there are many other examples from the Sunnah, one of which is the Hadeeth reported by Anas ibn Maalik in which he said:
“A man [a bedouin Arab] entered the mosque on the day of Jum’ah whilst the Prophet (Sallallaahu alaihi wa sallam) was delivering the Khutbah, and said: ‘O Messenger of Allaah, property is being destroyed, and the passages have become blocked, so supplicate Allaah to send rain down upon us.’ The Prophet (Sallallaahu alaihi wa sallam) raised his hands and then said: ‘O Allaah send down rain upon us; O Allaah send down rain upon us.’ The sky was clear without any cloud in it. The Prophet (Sallallaahu alaihi wa sallam) did not come down from his pulpit except that rain was flowing from his beard, may Allaah’s Salaat and Salaam be upon him. There was a downpour of rain for one week. Then on the succeeding Friday a man (the same man or someone else) entered the mosque and said: ‘O Messenger of Allaah, the houses are collapsing and wealth is being flooded, so supplicate Allaah to stop the rain for us.’ The Prophet (Sallallaahu alaihi wa sallam) raised his hands and began to say: ‘around us and not upon us,’ pointing with his hands, and he did not point to any direction except that it cleared.”  People then stepped out walking in the sunshine.
Many are the supplications which a person calls upon his Rabb (Allaah) to be answered are indeed responded to. This is a proof contained in what is experienced and perceived for the existence of Allaah, The Mighty and Majestic.
As for the Shar’ee proof for the existence of Allaah, then it is beyond being enumerated. Indeed the Qur’aan in its entirety as well as all of the affirmed Ahaadeeth in their legal and informative nature, proves the existence of Allaah, The Mighty and Majestic, as Allaah Says concerning the Sublime Qur’aan:
Had it been from other than Allaah, they would have surely found therein much discrepancy. [Qur’aan, soorat an-Nisaa’ (4): 82].
The belief in the existence of Allaah, is one of the matters that comprise(s) the belief in Allaah. As for the belief in Allaah’s Ruboobiyyah, Uloohiyyah, Names and Attributes, then a detailed discussion of these branches of Tawheed has preceded.
 Reported by al-Bukhaaree and Muslim. See Saheeh al-Bukhaaree,vol.2, no.467.
 Collected by al-Bukhaaree in his Saheeh, vol.2, no.55, and Muslim, vol.2, no.1955.
Translated by Dr Saleh as Saleh rahimahullaah
Source : Understanding Worship – Fiqh ul-‘Ibadah – Shaykh ibn Uthaymeen