Abu Dawud Book 2, Hadith Number 1496.
Chapter : The counting of praise and glorification of allah on pebbles.
Narated By Yusayrah, mother of Yasir : The Prophet (PBUH) 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.