What if someone rejects a sahih or hasan or weak hadith of prophet Muhammad that explains the Quran (I mean the hadiths that is doing the tafsir of the verses of the Quran) saying if prophet Muhammad said in front of him, he would accept it. Does this person goes outside of islam? (This person also believes that prophet Muhammad is a messenger of God).

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The mere rejection of the Hadeeth because of a given misconception, such as believing that it contradicts the Quran and the like, then this does not render a person a disbeliever.

However, he must follow the scientific approach in this regard, so he must accept what the scholars have accepted in this regard, and he should refer to them in regard to the explanation of the Hadeeth and its meaning; and he should reject whatever the scholars have rejected.

Each art has its specialist people, concerned with consolidating its foundations, among which is the science of Hadeeth.

Indeed, there are great and very prominent scholars who took special care about the Hadeeth of the Prophet and they clarified which Ahadeeth are authentic and which ones are Dha’eef (weak), so one must refer to them in regard to the Ahadeeth that they have accepted and whose which they have rejected.

Moreover, it should be noted that some of the Sunnah is an explanation of the verses of the Quran and clarifies its meaning, and some of it is independent in establishing some rulings.

In all of this, it is obligatory to follow the Prophet in compliance with the Saying of Allah (which means): {And whatever the Messenger has given you - take; and what he has forbidden you - refrain from.} [Quran 59:7]

For more benefit that it is compulsory to abide by the Sunnah and that it is like the Quran in legislation, please refer to Fatawa 92485, 91740, 88197 and 2740.

Allah Knows best.