The Hadith Qudsi [Reported by Muslim, Malik, at-Tirmidhi, Abu Dawud, an-Nasa'i and Ibn Majah]: I have divided the prayer between Myself and My servant.
Abu Hurayrah radiallahu 'anhu said: I have heard the Prophet sallallahu 'alayhi wa sallam say:
"Allah (the Mighty and Sublime) has said: I have divided prayer between Myself and My servant into two halves, and My servant shall have what he has asked for.
When the servant says: All praise belongs to Allah the Rabb of all the Worlds.
Allah (Mighty and Sublime is He) says: My servant has praised Me.
And when he says: The Most Gracious the Most Merciful.
Allah (Mighty and Sublime is He) says: My servant has extolled Me.
And when he says: Master of the Day of Judgement.
Allah says: My servant has glorified Me [and on one occasion He said: My servant has submitted to My power.]
And when he says: You alone do we worship and from You alone do we seek help.
He says: This is between Me and My servant, and My servant shall have what he has asked for.
And when he says: Guide us to the Straight Path, the Path of those whom you have favoured, not of those upon whom is your anger, nor of those who are astray.
He says: This is for My servant, and My servant shall have what he has asked for."
It is the delight of his heart, the pleasure of his eye and the joy of his soul that his Lord says about him: "My servant" three times. By Allah! If the fumes of desire and the darkness of the souls were not upon the hearts, they would have flown out of happiness and joy that their Lord, Maker and Deity says: "My servant has praised Me, My servant has extolled Me and My servant has glorified Me."
Then there would have been space in his heart to witness (the reality) of these three names which are the basis of all the Beautiful Names and these are: Allah, ar-Rabb and ar-Rahman ...
... When he says: Master of the Day of Judgement, he witnesses the glory which be fits none but the King, the Truth, the Manifest. So he witnesses a very powerful and compelling King, to Whom the whole creation has yielded, to Whom the faces are humbled, to Whom the creation humbles itself, and to Whom every possessor of power and honour submits to. He witnesses with his heart a King (Who is) upon the Throne of the Heaven ...
... And when he says: You alone do we worship and from You alone do we seek help, in which lies the secret of the creation and the affair, of the world and the hereafter, and which contains the loftiest of goals and the most excellent of means (of approach to Allah). The loftiest of goals is enslavement to Allah and the most excellent of means is His help and assistance. There is no deity deserving of worship in truth except He and there is no helper in His worship besides Him. Therefore, His worship is the greatest of goals and His aid is loftiest of means ...
... And these words contain the two types of tawhid and they are tawhid ur-rububiyyah and tawhid al-'uluhiyyah. Devotion and enslavement to Allah is comprised in the names ar-Rabb and Allah, so he is worshipped on account of His 'uluhiyyah, help is sought from Him on account of His rububiyyah and He guides to the Straight Path on account of His Mercy. The first part of the surah therefore, mentions His names Allah, ar-Rabb and ar-Rahman for the purpose of the one who seeks His worship, help and guidance. He is alone in being able to bestow all of that. There is no one who can aid others in His worship besides Allah and none can guide (to the Straight Path) except Him.
Then the caller realises with the speech of Allah: Guide us to the Straight Path, his extreme need for this (guidance). He is never in need of any other thing as much as he is for this guidance. He is need of it in every moment and at every glance of the eye ...
... Then Allah makes it clear that the people of this Guidance are those who have been specified with His favour, not of those upon whom is His anger, and they are the ones who know the Truth but do not follow it and not of those who are astray, and they are those who worship Allah without knowledge. Both of these groups share with each other in speaking about His creation, His command and His Names and Attributes without knowledge. The path of those upon whom is His favour and blessing is different from the people of falsehood with respect to knowledge and action.
When he finishes from this praise, supplication and tawhid he has been ordered to end it with "amin", which is a seal to (to his supplication) and to which the amin of the angels in sky coincides with. This amin is from the beautification of the prayer, just like the raising of the hands is a beautification of the prayer, a following of the sunnah, veneration of the command of Allah, worship by the hands and a sign of the movement from one pillar (of the prayer) to another.
(s) As-Salat wa Hukmu Tarikiha, p. 171-172