הלכה על ברכות 47:28

Likewise, regarding the hair on one’s head, the Sages teach: “A woman’s hair is erva” (Berakhot 24a). This refers to a married woman, and if her hair is not covered, a man may not recite sacred words in her presence (the specific of the laws of hair covering are detailed in Peninei Halakha: Collected Essays III 6:14-15).