From the scholia to Aristophanes, Acharnians 703-5, where he is mentioned: "for how is fitting that a man bent double, such as Thoukydides, should perish, entangled in the Skythian wilderness, at the hands of this Kephisodemos the loquacious advocate?" (web address 1).
Kephisodemos is unattested outside this passage, but Thoukydides can be none other than Th. Melesiou, probably the grandfather of the historian (theta 414) and certainly the principal opponent of Perikles (pi 1179 etc.); see generally OCD(4), under Thucydides(1).