Ibn Kathir (r) said...
So whosoever abandons the wise Shariah which was revealed upon Muhammad ibn Abdullah, the seal of the Prophets, and goes to other abrogated Shariah for judgement, he becomes a kafir. So how about the one who goes to al-Yaasiq (man made laws, the laws of the Tartars which mixed Shariah rulings with invented rulings) for judgement and gives it precedence over the Shariah of Muhammad (SAW). Whoever does this has become kafir by the Ijma of the Muslims
(Al-Bidayah Wan- Nihayah, Vol 13, pg 119)