“Water is the beginning and the end of everything.”
[οὕτος ἔφη] ἀρχὴν τοῦ παντὸς εἶναι καὶ τέλος τὸ ὕδωρ
So, we don’t really have anything by Thales, but this summary comes from the aspirationally named Refutation of all Heresies.
Thales fr. 20 (Hippolytus, Refutation of all Heresies. 1.1.1)