Is it obligatory to purify a garment thought to have been licked by a dog before praying in it?

It is forbidden to own a dog, except for a dog for farming, hunting, or herding. Anything other than that diminishes the reward of its owner by one or two Qirats daily. If a dog licks a vessel, it must be washed seven times, one of them with earth, if we are certain of its licking. However, if we are doubtful, washing is not obligatory.