Shaykh ul-Islaam Ibn Taymeeyah [رحمه الله] said:

❝Tawheed is the foundation of Eemaan, it is the statement of separation between the people of Paradise and the people of the Hellfire. It is the price of Paradise and no one’s Islaam is correct without Tawheed.❞

[Majmoo al-Fatawa, (24/235) | Translated By Abbas Abu Yahya Miraath al-Anbiyya]