The steadfast on tawheed

 The steadfast on tawheed. 

Tawheed التوحيد is the oneness of Allah. 

Allah says, 

Say: He is Allah, One and Only.

Surah 112;1.

Allah says, 

Aal-e-Imran 3:18

شَهِدَ ٱللَّهُ أَنَّهُۥ لَآ إِلَٰهَ إِلَّا هُوَ وَٱلْمَلَٰٓئِكَةُ وَأُو۟لُوا۟ ٱلْعِلْمِ قَآئِمًۢا بِٱلْقِسْطِۚ لَآ إِلَٰهَ إِلَّا هُوَ ٱلْعَزِيزُ ٱلْحَكِيمُ 

Allāh witnesses that there is no deity except Him, and [so do] the angels and those of knowledge - [that He is] maintaining [creation] in justice. There is no deity except Him, the Exalted in Might, the Wise.

The message of all messengers is worshipping Allah. 

Allah says, 

Al-Anbiya 21:25

وَمَآ أَرْسَلْنَا مِن قَبْلِكَ مِن رَّسُولٍ إِلَّا نُوحِىٓ إِلَيْهِ أَنَّهُۥ لَآ إِلَٰهَ إِلَّآ أَنَا۠ فَٱعْبُدُونِ 

And We sent not before you any messenger except We revealed to him that, "There is no deity except Me, so worship Me."

Tawheed is the key to jannah. 

As reported, 

Narrated Ibn `Abbas:

When the Prophet (ﷺ) sent Mu`adh to Yemen, he said to him, "You are going to a nation fromthe people of the Scripture, so let the first thing to which you will invite them, be the Tauhid of Allah. If they learn that, tell them that Allah has enjoined on them, five prayers to be offered in one day and one night. And if they pray, tell them that Allah has enjoined on them Zakat of their properties and it is to be taken from the rich among them and given to the poor. And if they agree to that, then take from them Zakat but avoid the best property of the people."

Sahih al-Bukhari 7372.

So the matter of tawheed is first before the solaah. 

Narrated Mu’adh bin Jabal ::

The Messenger of Allah (ﷺ) as saying: If anyone's last words are "There is no god but Allah" he will enter Paradise.

Sunan Abi Dawud 3116.