From the Virtues of Tawheed is that.....

[Muwaahid]

And from the virtues of Tawheed is what Shaykhul Islaam Muhammad Bin Abdul Wahhaab [rahimahullah] said, and [when] you know that this is Tawheed , that which Allah has made MORE obligatory than the [obligatory] Salah and Saum [obligatory fasting], and that Allah will ONLY FORGIVE THOSE WHO COME WITH IT ON YAUMAL QIYAAMAH, and He WILL NOT FORGIVE THOSE WHO ARE IGNORANT OF IT,EVEN IF HE WAS FROM AMONGST THOSE WHO WERE MOST EXCESSIVE IN WORSHIP, and know that it is indeed Shirk [with respect to calling upon the righteous seeking intercession] of which Allah will NOT FORGIVE the perpetrator of it and it is considered with Allah more greater [in evil] than Az-Zinaa [fornicating] and killing a soul, whilst the perpetrator of shirk only desires with it attaining closeness & nearness to Allah."