Following a Math-thab and Praying Behind an Imam From A Different School
3-6-2002 | IslamWeb
Question:
What is the ruling regarding following a certain Madhab? Can I pray behind a person who prays according to the government rule if we know how to pray according to the Prophetic tradition?
Answer:
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.
There is no harm in offering prayers behind any Imam who performs valid prayers; i.e. who performs the prayers while fulfilling all prerequisites, integral parts of the prayers, and free from the violations of a prayer regardless of the differences between the Imam and the congregation (people led in prayer). If it is made a condition that an Imam and people led in prayer should be followers of the same school the people will split up into groups and everyone will perform according to his opinion and opinion of his School of jurisprudence and it becomes impossible to make a congregation; then there is no doubt that this would be totally contrary to the basic purpose for which the congregation is legislated and contrary to the objectives of the Sharee'ah as well.
Imam Ibn Taymiyyah 
was asked about the validity of the prayer performed behind a person who follows a different school of jurisprudence: What about an Imam who does something in prayer which is valid according to his opinion while the person led in prayer believes otherwise? He replied: “The prayer is correct if it is performed behind a person who follows another school of jurisprudence. The companions of the Prophet the Taabi'een (the generation that followed the companions), their followers and the four Imams used to perform prayers behind others even though they differed in some matters with the person who led the prayer, and according to them, the matters were among the nullifiers of the prayer.” This brief quotation is from a long answer issued by Imam Ibn Taymiyyah . He emphasized the point that performing prayers behind an Imam who belongs to a different school of jurisprudence is valid even if something he does invalidates the prayer in the opinion of the person led in prayer.
Allah knows best.