Al-Tabari reported: A man asked Ibn Abbas, may Allah be pleased with him, about this verse, “Whoever does not judge by what Allah has revealed, they are the unbelievers,” (5.44). The man said, “Has one who does this committed unbelief?” Ibn Abbas said, “It is a type of unbelief if he does that, but it is not like one who disbelieves in Allah, the Last Day, and so on.”
Source: Tafsīr al-Ṭabarī 5:44
عن الطبري قَالَ رَجُل لِابْنِ عَبَّاس رضي الله عنه فِي هَذِهِ الْآيَات وَمَن لَّمْ يَحْكُم بِمَا أَنزَلَ اللَّهُ فَأُولَٰئِكَ هُمُ الْكَافِرُونَ قَالَ فَمَنْ فَعَلَ هَذَا فَقَدْ كَفَرَ قَالَ اِبْن عَبَّاس إِذَا فَعَلَ ذَلِكَ فَهُوَ بِهِ كُفْر وَلَيْسَ كَمَنْ كَفَرَ بِاَللَّهِ وَالْيَوْم الْآخِر وَبِكَذَا وَكَذَا
5:44 تفسير الطبري