In the Holy Qur'an

And keep up prayer in the two parts of the day and in the first hours of the night (11:114)

In the Traditions

The Prophet said, "The most noble of my community are those who carry the Qur'an in their hearts, and those who stand in prayer at night."

THe Prophet said, "Surely Allah will place light in the heart of the slave who retires with his Master in the deepest darkeness of the night, coversing with Him in prayer; and if he says, 'O Lord' then Allah will call out to him saying, 'At your service, my slave, whatever you ask of Me, i will surely give you and if you reply on Me then i will be enough for you'.
Then He will say to the Angles, "My Angels, look at My slave - he retires alone with Me in the deepest darkness of the night while the misguided are playing and the negligent are sleeping, Bear witness that i have forgiven him."