Prayer (Kitab Al-Salat): Detailed Injunctions about Witr
Dawud :: Book 8 : Hadith 1520
Narrated Asma' daughter of Umays:
The Apostle of Allah (peace_be_upon_him) said to me: May I not teach you phrases which you utter in distress? (These are:) "Allah , Allah is my Lord, I do not associate anything as partner with Him."