Shaikh Saleh al-Fawzaan حفظه الله: The one who denies al-Ijmaa` is a Kaafir.

بـسـم الله والحـمـد لله والـصلاة والـسـلام عــلى رسـول الله، وبـعـد

Question: Your Excellency! Some of the scholars say: “Al-Ijmaa` (consensus of the scholars) is not a proof, it is just a way of consultation as there is no consensus at all except to look for the evidence in the Text (i.e. the Book and the Sunna). Moreover, the claim – in many issues – that there is a consensus regarding them is a matter of doubt.” What do you have to say about this?

Shaikh al-Fawzaan حفظه الله answers: Absolute denial of al-Ijmaa` tantamount to Kufr and apostasy from Islaam. Absolute denial of al-Ijmaa` - that the person says that there is no consensus at all – then he is a Kaafir. As for the one who denies some aspects of al-Ijmaa`, then this is debatable, and his words should be analyzed; is he correct or is he incorrect?

This is because there are issues where it is claimed that there is Ijmaa` regarding them, but (in reality) no consensus is reached in these regards. But as for the one who absolutely denies the consensus and says that they are not proof, then he is a Kaafir. He is a Kaafir in Allaah عز وجل, because al-Ijmaa` is there, and all praise be to Allaah. The scholars have consensus regarding `Aqeedah (Creed), they have consensus regarding Tawheed of Allaah عز وجل, and they have consensus regarding the prohibition of Ribaa (Usury and interest), and they have consensus regarding the impermissibility of Zinaa (adultery and fornication), and they have consensus regarding the issues of Deen which are known due to necessity. The one who absolutely rejects the consensus is a Kaafir. As for the one who rejects some aspects of it, then there might be a reason, so he is to be looked at and examined.

Source:
http://www.alfawzan.af.org.sa/node/2387