Q 24: What is the textual proof from the Glorious Qur'an and the Prophetic Sunnah that a Muslim must be sincere?
A--- Allah, the Almighty says: {Surely the religion (i.e. the worship and the obedience) is for Allah only}. (Az-Zumar, 3)
Allah says: {So worship Allah (Alone) by doing religious deeds sincerely for Allah's sake only}. (Az-Zumar, 2)
Prophet Muhammad (Peace be upon him) says: "The luckiest person who will have my intercession on the Day of Resurrection will be the one who said sincerely from the bottom of his heart 'None has the right to be worshipped but Allah' ". (Sahih Bukhari)
He (Peace be upon him) said also, "Allah has forbidden the (Hell) fire for those who say, 'None has the right to be worshipped but Allah' for Allah's sake only". (Bukhari and Muslim)
Reference: 200 FAQ On Muslims Belief - Shiekh Hafiz Ibn Ahmed Al Hakami