I am sure this is a problem we have all encountered. A presumably Vedic citation is given in a text, and it cannot be traced. But when such citations are given in such an ancient and authoritative text as Śabara’s commentary on the Mīmāṃsā Sūtras, however, they raise an issue. What do we make of such citations, which, I am sure, Śabara knew to be Vedic. Some of these can be traced, as Agrawal (Mīmāṃsā-Uddharaṇa-Koṣa) does, to extant texts such as the Śatapatha, but the wording is only approximate. 

So we have 

traced to ŚBr 3.1.2.16 (Agrawal give 3.2.1.16, which is an error), but there we have: सुवाससा एव बुभूषेत् (rest omitted). 

I wonder whether anyone has thought about this. Thanks.

Patrick