Tafsir of Al-Ma'aarij 70:23

Those who with their Salah are Da'imun; It has been said that this means they guard its times and the elements obligatory in it. This has been said by Ibn Mas`ud, Masruq and Ibrahim An-Nakha`i. It has also been said that it means tranquility and humble concentration (in the prayer). This is similar to Allah's statement,

قد أفلح المومنون

الذين هم فى صﻼتهم خشعون

Successful indeed are the believers. Those who with their Salah are Khashi`un. (23:1-2) This was said by `Uqbah bin `Amir. From its meanings, is the same terminology used to describe standing (still) water (Al-Ma' Ad-Da'im). This proves the obligation of having tranquility in the prayer. For verily, the one who does not have tranquility (stillness of posture) in his bowing and prostrating, then he is not being constant (Da'im) in his prayer. This is because he is not being still in it and he does not remain (in its positions), rather he pecks in it (quickly) like the pecking of the crow. Therefore, he is not successful in performing his prayer. It has also been said that the meaning here refers to those who perform a deed and are constant in its performance and consistent in it. This is like the Hadith that has been recorded in the Sahih on the authority of `A'ishah that the Messenger of Allah said,

أحب اﻻعمال إلى الله أدومها وإن قل

The most beloved deeds to Allah are those that are most consistent, even if they are few. Then Allah says,

والذين في أموالهم حق معلوم