A few references in Revelation, depending upon one's interpretation. I hold to the early preterist perspective.
Rev 14:2: And I heard a voice from heaven, as the voice of many waters, and as the voice of a great thunder: and I heard the voice of harpers harping with their harps:
I checked the Greek text and it has kithara which means lyre or lute. Kithara via Latin cithara is the source of the word guitar.
The kithara was triangular-shaped and had 7 strings.