Ta'dîl-i Arkân

Hadrat Imâm-i Rabbânî says in the 87th letter of the second volume of his Maktûbât:

In other words, each Muslim's first duty is to obey the Sharî'at and refrain from the things which the Sharî'at dislikes, from the bid'ats.

If a person uncovers an abandoned, forgotten sunnat, he will get the thawâb of a hundred martyrs. Then, just imagine how much thawâb there is in resuscitating a fard or wâjib. Therefore, one should be careful about the ta'dîl-i arkân when performing namâz. In other words, at the rukû, at the sajda, at the qawma and at the jalsa, each limb should become motionless and remain so for a while; most of the savants of the Hanafî Madhhab said that this was wâjib. Imâm-i Abû Yûsuf and Imâm-i Shâfi'î said that it was fard. The majority of Muslims have been neglecting this. A person who reveals this one deed will be given much more thawâb than that which is given to a hundred martyrs who have fought and given their lives in the way of Allah. This same rule applies to all the rules of the Sharî'at. That is, a person who teaches one of the halâls, harâms, makrûhs, fards, wâjibs and sunnats and has it obeyed will get the same amount of thawâb.

Hadrat Imâm-i Rabbânî says in the 69th letter of the second volume of his Maktûbât:

Recently, most people have been slack in performing namâz. They have been slighting the tumânînat and the ta'dîl-i arkân. For this reason, I have to warn you, my beloved ones, about this matter. Listen well! Our Prophet 'sall-Allâhu alaihi wa sallam' declared: "The worst thief is the person who steals from his own namâz." When he was asked, "O Rasûlallah! How can a person steal from his own namâz?" he said, "By not doing the rukû and sajda of the namâz properly." At some other time he declared: "Allâhu ta'âlâ does not accept the namâz of a person who does not bring his waist into its proper position and remain so for a while in rukû and sajda." Once, upon seeing a person not doing the rukû and the sajda properly while performing namâz, our Prophet 'sall-Allâhu alaihi wa sallam' said, "Aren't you afraid you may die in some other religion than Hadrat Muhammad's 'alaihissalâtu wassalâm' dîn because you perform your prayers of namâz in this manner?" Once again, he said, "When performing namâz, if you do not straighten up your body completely after the rukû', if your each limb does not rest at its position for a while when you are standing, your namâz will not be complete." Once again, he said, "Unless you sit upright between the two sajdas your namâz will remain incomplete." One day, upon seeing someone not observing the rules and rukns of salât, not standing upright after the rukû' and not sitting between the sajdas, our Prophet 'sall-Allâhu alaihi wa sallam' said, "If you go on performing your prayers of namâz in this manner, on the Day of Resurrection you will not be said to be of my Ummat." At some other time he said, "If you go on in this manner and die, you shall not have died in the religion of Muhammad (alaihissalâm)."