Prayer (Kitab Al-Salat): Detailed Injunctions about Witr
Dawud :: Book 8 : Hadith 1496
Narrated Yusayrah, mother of Yasir:
The Prophet (peace_be_upon_him) commanded them (the women emigrants) to be regular (in remembering Allah by saying): "Allah is most great"; "Glory be to the King, the Holy"; "there is no god but Allah"; and that they should count them on fingers, for they (the fingers) will be questioned and asked to speak.