Prayer (Kitab Al-Salat): Detailed Injunctions about Witr
Dawud :: Book 8 : Hadith 1474
Narrated An-Nu'man ibn Bashir:
The Prophet (peace_be_upon_him) said: Supplication (du'a') is itself the worship.
(He then recited:) "And your Lord said: Call on Me, I will answer you" (xI.60).