Deuteronomy 6:7 "And you shall talk of them when you sit in your
house, and when you travel on the road, and when you lie down and when
you rise up"
HaShem commanded us to recite the Shema twice every day, in the
morning and the evening.
When our Sages arranged the content of the Siddur, they included the
Shema in the morning and evening prayers.
The Hebrew word Shema - gives us a hint about our three daily prayers.
The first letter Shin stands for Shacharit - the morning prayers.
The second letter Mem stands for Minchah -, the afternoon prayers.
The third letter Ayin stands for Arvit (Maariv) - the evening prayers.