My question is regarding the grade of hadiths; there are some hadiths concerning which Hadith scholars differ in grading them; like, there is a hadith in which Prophet Muhammed, sallallaahu ‘alayhi wa sallam, said that knowledge is obligatory upon every muslim, so on this hadith, Hadith scholars differed; some say that it is an authnetic hadith, and some say that it is a weak hadith. So whom should we follow in such case?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger. 

The duty of the lay Muslim who does not have the ability to weigh the statements of scholars against each other - whether pertaining to declaring Ahaadeeth authentic or weak or any other field of religious knowledge - is to ask a scholar whom he trusts to be religious and knowledgeable. He then acts upon what he tells him, in compliance with the command of Allaah, The Exalted: {So ask the people of the message if you do not know.} [Quran 16:43]

It is sufficient for him to follow a scholar whom he believes to be trustworthy in that branch of knowledge. There is no doubt that early scholars are worthier of being followed in this regard given their chronological proximity to the original sources of Islamic knowledge and the fact that they were more knowledgeable and proficient in their respective disciplines.

Allaah knows best.