What is the ruling on the prayer of someone who swam in a pool into which a dog entered and drank, or whose wet clothes or towels were touched by dog hair? And does the purification from the effect of the dog become void over time as a result of repeated ablution and ghusl?

"Water does not become impure if it is more than two qullas and its properties are not altered by impurity. A swimming pool or a swimming pond is usually like this, so the lapping of a dog in it does not harm it.

Clothes do not become impure by touching dog hair if both are dry, or if one of them is wet, based on the opinion that dog hair is pure. This is the view preferred by Sheikh Al-Islam Ibn Taymiyyah, and that the preponderant opinion is the purity of all hair except for saliva.

Accordingly, if the dog's hair is wet and touches a garment, there is nothing upon the person, and this is the madhhab of the majority of jurists, because the default state of all substances is purity."