The Family Of Imran - 3:191
"Those who remember Allah while standing, sitting and lying on their sides. They think about the structure and creation of the heavens and the earth: "Our Lord, You did not create them in vain, You are exalted, protect us from the torment of the Fire."
Related:
The Women/4:103
When you have finished the prayer, remember Allah standing, sitting and lying down. When you are safe, observe the prayer. Prayer has been made obligatory on the believers at fixed times.