By Shaykh Hamood Ibn Uqla ash Shuaybee
From the conditions of the correctness of Tawheed is kufr bi Taghut and there is no Imaan except after kufr bi Taghut outwardly and inwardly.
فَمَن يكْفُرْ بِالطَّاغُوتِ وَيؤْمِنْ بِاللَّهِ فَقَدِ اسْتَمْسَكَ بِالْعُرْوَةِ الْوُثْقَى لا انفِصَامَ لَهَا وَاللَّهُ سَمِيعٌ عَلِيمٌ
Whoever disbelieves in Taghut and believes in Allah, then he has grasped the most trustworthy handhold that will never break. And Allah is the All Hearer the Allah Knower
Surat Al Baqarah 2:256
And Allah Said:
وَلَقَدْ بَعَثْنَا فِي كُلِّ أُمَّةٍ رَّسُولاً أَنِ اعْبُدُوا اللَّهَ وَاجْتَنِبُوا الطَّاغُوتَ
And verily We have sent among every Ummah a Messenger (proclaiming) worship Allah and avoid Taghut
Surat An Nahl 16:36
And the Messenger of Allah salallahu alayhi wa salam said:
من قال لا إله إلا الله وكفر بما يعبد من دون الله حرم ماله ودمه وحسابه على الله
Whoever says there is no god but Allah and disbelieves in what is worshipped other than Allah, then his blood and wealth is haram and his reckoning is with Allah.
Shaykh Muhammad Ibn Abdul Wahhab said: “Whoever worships Allah day and night, then makes Dua to a prophet or a wali in the grave then he has taken two gods and and he has not in fact testified that there is no god but Allah, as a god is the one who is made dua to like the Mushrikeen done at the grave of Zubayr or Abdul Qadir or other than them. And whoever sacrifices for Allah one thosand times, then sacrifices to a Prophet or other than him, then he has taken two gods as Allah Said:
قُلْ إنَّ صَلاتِي وَنُسُكِي وَمَحْياي وَمَمَاتِي لِلَّهِ رَبِّ الْعَالَمِينَ
Say verily my prayer and my sacrifice and my living and my dying is for Allah, the Lord of the Alamin.
Surat Al Anaam 6:162