Minchat Shai at the beginning of Shemot 20 describes a version of the text of the Decalogue where the tzadi in the word תרצח has both a kamatz and a patach under it. He explains that the kamatz belongs to what we call ta’am elyon, and the patach to ta’am tachton.
It seems that this version does not exist in some of the oldest manuscripts. See e.g. Mechon Mamre’s version of ta’am tachton for Shemot 20, where the tzadi has a kamatz. Or here from the Leningrad Codex, where no patach appears.
So, where is the first attestation of the version described by Minchat Shai, with a patach under the tzadi?