Who are David and Sibyls in the first stanza of Dies Iræ?

The first stanza of the medieval Latin hymn *Dies Iræ* is as follows: > Dies iræ, dies illa > > Solvet sæclum in favilla, > > Teste David cum Sibylla. While the beginning is clearly taken from the prophet Zephaniah : > Dies iræ dies illa, dies tribulationis et angustiæ, dies calamitatis et miseriæ, dies tenebrarum et caliginis, dies nebulæ et turbinis (Zef 1:15) I am wondering to whom the names the last line refer and what specific testimonies are ment. For **David** the Old Testament king of Israel comes to mind – but is there any specific testimony concerned with the day of wrath? For ***Sibylla***, I do not even find her name in the bible. Wikipedia refers to some ancient greek oracles ; but why should they be named in a christian hymn. Is there a definitive interpretation? Who are these two and can their specific testimonies identified?