Making the Deen solely for Allāh is the Foundation of the Deen and the axis around which it revolves; and this is the Tauheed with which Allāh sent all the Messengers (Alahym Salam), and with which He revealed the Books, and to it the Prophets called, upon it they strove, it is it they commanded of, and towards it they induced the people.
Allāh the Mighty said:
"[Verily, we have sent down the Book to You (O Muhammad) in truth:] so Worship Allāh (Alone) by doing religious deeds sincerely for Allāh's sake only, (and not to show-off, and not to set up rivals with Him in worship). Surely, the Religion (i.e. the worship and the obedience) is for Allāh only." [Zumar: 2-3].
And He the Mighty said:
"And they were commanded not, but that they should Worship Allāh, and Worship none but Him alone (abstaining from ascribing partners to him), and perform As-Salāt and give Zakāt." [Al-Bayyinah: 5].
And Allāh said to His Prophet (Sal-Allaahu 'alayhe Wa Sallam):
"Say (O Muhammad) "Allāh alone I worship by doing religious deeds sincerely for his sake Only and not to show-off, and not to set up rivals with Him In worship." So worship what you like besides Him." [Az-Zumar: 14-15].
And He the Mighty said:
"Say (O Muhammad): "Verily, My Salāt (prayer), My sacrifice, My living, and My dying are for Allāh, the Lord of the 'Alamīn (mankind, jinns and All that exists). "He has no partner. and of this I have been commanded, and I am the first of the Muslims." [An'aam: 162-163].
That is which is like the foundation of a house.
Ibn Al-Qayyim (Allāh's mercy on him) said:
'Whoever wants that his house should go up then he ought to get a good foundation for it, get it well-arranged and should have utmost concern for it; for a house to go up depends on the capacity of the strength of its foundation. (Human) deeds and statuses is the house, its foundation is Imaan [Faith]
An intelligent (Muslim) will always have concern for his foundation and its suitability; but an ignoramus will (continue) to build his house on no (strong) foundation; thus there is no guarantee for his house from falling down. Allāh the Mighty said:
"Is it then he, who laid the foundation of his building on piety to Allāh and his good pleasure, better, or He who laid the foundation of his building on an undetermined brink of a precipice ready to crumble down, so that it crumbled to pieces with Him into the Fire of Hell." [Taubah: 109]. [End of quote from Ibn al-Qayyim (see Al-Fawaaid, p. 204)].
I say (that is, Shaykh Ramadaanee): That is because the Verse was revealed with regard to the hypocrites who built a mosque for observance of Salat but when they came up with this great and rewarding act (of building a mosque) their hearts were devoid of sincerity (to Allāh) therefore their (this lofty) deed availed them of nothing rather with it, they were thrown into the fire of hell as it was mentioned in the Verse.
Then he (Ibn Qayyim), may Allāh show mercy to him, said:
'This foundation (which is the ) has two matters (connected to it):
One: The correctness of knowledge with regard to Allāh and His Command; so also His Names and Attributes.
Two: Freeing accepting Allāh and His Messenger from other than that.
This is the strongest foundation upon which a believer can build his edifice; it is from whence the house will go up as he might deem. Therefore (O Muslim,) select the best foundation, guard its power and always be on guard
' [Al-Fawaaid p. 204].
Source: Sittu Durar Min Usuuli Ahlil'Athar [Six Pearls from the Foundations of People of Tradition]
By Ash-Shaykh Abdul-Malik bn Ahmad Al-Mubārak Ramadaanee Al-Jazāirī - hafidha-huLlaah