Alladhina hum ala salatihim daimuna
Those who are steadfast in their salat
The following is an excerpt of commentary on this ayah, from Tafsir Ibn Kathir:
It has been said that this means they guard its times and the elements obligatory in it. This has been said by Ibn Masud, Masruq and Ibrahim An-Nakha'i. It has also been said that it means tranquility and humble concentration. 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 Aisha that the Messenger of Allah said, "The most beloved deeds to Allah are those that are most consistent, even if they are few"