Al-Haafidh Ibn Katheer said:
"So whoever leaves the clear Sharee'ah, which
was revealed to Muhammad Ibn Abdullah, the
Seal of the Prophets, and takes the Hukm to
other than it from the laws of Kufr which are
abrogated, he has disbelieved. So what about
the one who takes the Hukm to the
'Yasaaq' (the law of the Tartars which mixed
Sahree'ah rulings with invented rulings) and
puts it before it?! Whoever does that, he has
disbelieved by the Ijmaa' of the Muslims."
– "Al-Bidaayah wa Nihaayah", Vol. 13/ 119