‘Umar bin al-Khaṭṭāb

'Umar bin al-Khaṭṭāb (may Allāh be pleased with him) said: “If one of you knew what is for him (in reward) in saying to his brother: Jazākallāhu khayran (may Allāh reward you with good), you would increase in saying it to one another.”

'Umar bin al-Khaṭṭāb (may Allāh be pleased with him) wrote to his deputies, saying: “Verily, the most important of your affairs to me is the prayer (ṣalāh). Whoever maintains it and watches over it will maintain his religion. Whoever wastes it will be even more wasteful of other things.”