Pious Reverence (Khushu)
The outward prerequisites of prayer are regulated by Islamic jurisprudence (Fiqh). A prayer that does not observe the regulations set by Islamic law is not acceptable. However, a prayer that is not done in pious reverence is not a respected one. Therefore, a prayer should combine the outward regulations and the inner ones that adorn the heart. Adornment of the heart can be accomlished by actualizing the secret of purification. It is said in the Qur’an:
“But those will prosper who purify themselves.” (Al-A’la (7: 14).
This spiritual cultivation is so important for prayer. Allah does not mention the obligatory (fardh), the necessary, (wajib), and the number of units of prayer, but rather repeatedly states the importance of pious reverence, sincerity, and peace of mind. The importance of these states covers our entire life. So, the spiritual side of prayer is the most important component that the praying person should observe. It is said in the Qur’an:
“The believers must (eventually) win through: those who humble themselves in their prayers...” (Al-Muminun 23: 1-2).
The Prophet (pbuh) says:
“Whoever makes ablution accordingly, and prays in time, and bows down and prostrates in pious reverence, his prayer arises as a luminous light and calls to the praying person: ‘ –May Allah save you as you observed my details!’ And whoever does not make ablution accordingly, and does not pray in time, and does not bow down and prostrate in pious reverence, his prayer arises as a dark object and calls to the praying person: ‘ – May Allah waste you as you wasted me!’ So that the prayers, following Allah’s decree, goes to a place and then comes back to slap the face of the praying person.” (Tabarani).
Bahaaddin al-Naqshiband was once asked:
“ – How does a servant achieve pious reverence in prayer?”
He answered:
“ – There are four prerequisites for that:
1.Legitimate earning,
2.Remaining vigilant during ablution for prayer,
3.Feeling the presence of Allah as one begins prayer by saying “Allah is Great!”
4.Feeling the presence of Allah all the time; remaining in peace and tranquility, and obeying Allah after prayer.”
Pious reverence in prayer is so important that the servant is treated by Allah accordingly. The Prophet (pbuh) says:
“When a person completes his prayer, one tenth of its reward is given to him; or he is given one ninth; or one eighth; or one seventh; or one sixth; or one fifth, or one fourth, or one third, or half...” (Abu Dawud, Salat, 124).
“Many persons cannot get one sixth, or even one tenth, of the reward for his prayer. He only gets the part he does in pious reverence.” (Abu Dawud, Nesai).
Namely, the servant gets reward for only the prayer he does in pious reverence.
Those who do prayers in sincerity to perform prayers adequately, and thereby to commit themselves to Allah, and they engage themselves only with prayer, and perform prayer in order that they open to spiritual accomplishments. They fix their eyes on the point to which they prostrate, and sensing that they are under Divine scrutiny, they go into rapture in spiritual pleasure.
This is, of course, the condition of the sincere servants with a sound heart. Namely, pious reverence is the fruit of sincerity. Then, sincerity provides the servant with pious reverence, and it advances him to high degrees before Allah as well as providing Divine protection. The Prophet (pbuh) says in this regard:
“Good news is for those who are guided into the right way. It is just because of them strong troubles vanish.” (Fadail-i Amal).
source
http://www.namazzamani.net/english/pious_reverence.htm