Ruling 118

If someone knows that one of two utensils or one of two items of clothing that he uses has become impure but does not know which one, he must avoid both of them [i.e. they are ruled to be impure]. However, if, for example, one does not know whether it is his own clothing that has become impure or clothing that he does not have any right of disposal over and which is the property of someone else, it is not necessary for him to avoid it [i.e. it is ruled to be pure].