All Praise to Allah Subhanahu wa Taala and blessings of Him be on Prophet Muhammad peace be upon him His family and companions
It was narrated from Abu Hurairah that : The Prophet said "I do not
want to hear of anyone of you who, upon hearing a Hadith narrated from
me, says while reclining on his pillow: 'Recite Qur'an (to verify this
Hadith).' (Here the Propher SAW said) Any excellent word that is said,
it is I who have said it." [How can you reject what I have said?](Ibn
majah)
According to Fardh i'tiqadi (Fard belief) ,If Daleel is Qati (Proved from Quran and Mutawatir Hadith undoubtfully) then rejector of Fard I'tiqati would be Kafir(Disbeliever) and there is a consensus among all four school of thoughts that if issue is clear enough to all (like Prayers,Ruku,Sajdah) then he would be considered as Kafir (Disbeliever) and if some other person doubt about Kufr of rejector of Fard belief he would be considered as Kafir too.
And when there came to them a Book from Allah confirming that which
was with them - although before they used to pray for victory against
those who disbelieved - but [then] when there came to them that which
they recognized, they disbelieved in it; so the curse of Allah will be
upon the disbelievers.How wretched is that for which they sold
themselves - that they would disbelieve in what Allah has revealed
through [their] outrage that Allah would send down His favor upon whom
He wills from among His servants. So they returned having [earned]
wrath upon wrath. And for the disbelievers is a humiliating
punishment.(Quran 2:89-90)
Allah Swt and His Messenger knows best.