Must the type of animal be specified when hiring it for drawing water?

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It is likely obligatory to specify the type of animal when drawing water because of differences in the purity of their excrement and body. Some animals, such as horses and cattle, have excrement and bodies that are considered pure without dispute. Others, such as mules and donkeys, have impure excrement, and variation exists regarding the impurity of their bodies, which might contaminate the drawn water or the bucket, thus nullifying the purpose of the contract.