VIII. About baptism by heretics.
For they who have received baptism from heretics, not having been previously baptized, are to be confirmed by imposition of hands with only the invocation of the Holy Ghost, because they have received the bare form of baptism without the power of sanctification. And this regulation, as you know, we require to be kept in all the churches, that the font once entered may not be defiled by repetition, as the Lord says, “One Lord, one faith, one baptism.” And that washing may not be polluted by repetition, but, as we have said, only the sanctification of the Holy Ghost invoked, that what no one can receive from heretics may be obtained from Catholic priests. This letter of ours, which we have sent in reply to the inquiries of the brotherhood you shall bring to the knowledge of all your brethren and fellow bishops of the province, that our authority, now that it is given, may avail for the general observance. Dated 21st March, in the consulship of Majorian Augustus (458).
Source: Letters (New Advent)