Salaah is the greatest of the practical pillars of Islam, and khushoo' in prayer is required by sharee'ah. Allah says in the Quran: "Successful indeed are the believers, those who offer their salaah (prayers) with all solemnity and full submissiveness." [alMu'minoon 23:1-2] - i.e., fearing Allah and in a calm manner. Khushoo' means calmness, serenity, tranquility, dignity and humility. What makes a person have this khushoo' is fear of Allah and the sense that He is always watching. Khushoo' means that the heart stands before the Lord in humility and submission.