Source, in full
This is often attributed to Hippocrates but does not appear in the Hippocratic corpus. See Diana Cardenas, "Let not thy food be confused with thy medicine: The Hippocratic misquotation", e-SPEN: The European e-Journal of Clinical Nutrition and Metabolism XXX:6 (October 2013). Cardenas here in turn cites Jaques Jouanna.
Notes from Wikiquote
As Cardenas self quotes from the Hippocratic corpus (On Food, s:el:Περί_τροφής#pXIX.), the source of the misquotation might be: “in food medicine for the best, in food medicine for the bad, bad and good related to something” («ἐν τροφῇ φαρμακείη ἄριστον, ἐν τροφῇ φαρμακείη φλαῦρον, φλαῦρον καὶ ἄριστον πρός τι»).