What is the evidence for stipulating the certainty in the Quran and the Prophetic Sunnah?

What is the evidence for stipulating the certainty in the Quran and the Prophetic Sunnah?

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Allah said ( interpretation of the meaning ):

{ The believers are only the ones who have believed in Allah and His Messenger and then doubt not but strive with their properties and their lives in the cause of Allah . It is those who are the truthful.}

Quran ( 49 : 15 )

I bear testimony to the fact that there is no god but Allah, and I am His messenger. The bondsman who would meet Allah without entertaining any doubt about these (two fundamentals) would enter heaven.

[Muslim]

And said to Abu Huraira

"Go and give glad tidings of Jannah to anyone who testifies 'La ilaha illallah (There is no true god except Allah),' being whole-heartedly certain of it" .

[Muslim]