Shaykh Muhammad Saalih Ibnul ‘Uthaymeen:

"...So the Prophet, sallaahu 'alayhi wa-sallam, explained the whole of the Religion of Islaam through his sayings , his actions and his tacit approvals, either to begine with, or in response to a question.

And the greatest of all affairs which he made clear (through his sayings, action and tacit approvals), is Tawheed."

Reference: Explnation of the Three Fundamental Principles of Islaam p 224.

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