All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The preponderant opinion of the scholars is that the saliva of a dog is impure but its hair and skin are pure.
Therefore, if a dog touches a praying person with its skin or hair, then this does not harm provided there is no other impurity on its body.
If there is any impurity on the dog’s body and it touches a praying person, his prayer becomes null since being away from impurity during the prayer is compulsory. Likewise, the prayer becomes null if the saliva of a dog touches a praying person. In this case, one should wash his/her body or clothes seven times; one of which should be with soil.
Allaah Knows best.