Diogenes Laertius, Lives of the Philosophers 9.1.2-4:
“Hermippus says that Heraclitus asked the doctors if one could, by emptying his bowels, dry out the moisture [from dropsy]. When they denied that it could be done, he laid himself out in the sun and ordered some boys to smother him in cow shit. On the next day, he died stretched out in that way, and was buried in the agora. Neanthes of Cyzicus says that because he was unable to get it off, the shit remained on him; because of this change in his appearance, he was unrecognized, and was eaten by dogs.”
Ἕρμιππος δέ φησι λέγειν αὐτὸν τοῖς ἰατροῖς εἴ τις δύναται <τὰ> ἔντερα κεινώσας τὸ ὑγρὸν ἐξερᾶσαι: ἀπειπόντων δέ, θεῖναι αὑτὸν εἰς τὸν ἥλιον καὶ κελεύειν τοὺς παῖδας βολίτοις καταπλάττειν: οὕτω δὴ κατατεινόμενον δευτεραῖον τελευτῆσαι καὶ θαφθῆναι ἐν τῇ ἀγορᾷ. Νεάνθης δ᾽ ὁ Κυζικηνός φησι μὴ δυνηθέντ᾽ αὐτὸν ἀποσπάσαι τὰ βόλιτα μεῖναι καὶ διὰ τὴν μεταβολὴν ἀγνοηθέντα κυνόβρωτον γενέσθαι.