Two Mishnayos in Nedarim, 3:11 and 9:10, contain quotes from the eulogy of Shaul by Dovid . Both quotes are tangents, they don't directly relate to each other in context, and neither is really integral to the message of the mishnah. As far as I know there are no other quotes from this Kinah (eulogy) anywhere in mishnayos, though there is one in the Gemara in Avodah Zara 25a.
This seems too unlikely to just be a coincidence. Does anyone develop a deeper connection between Nedarim and Kinas David?