In the Holy Qur'an

Say: call upon Allah or call upon the Beneficent; whichever you call upon, He has the best names. (17:110)

In the Traditions

The Prophet said, "As for prayer, it is the essense of worship."

O He who

gives to a servant
who asks from Him,

takes him to his wish
when he expectantly hopes for what is with Him,

brings him near and close
when he approaches Him,

covers over his sin and cloaks it
when he shows it openly,

and satisfies and suffices him
when he confidence in Him!

Excerpt of The Whispered Prayer of the Thankful
by Imam Zainul Abideen