הלכה על שבת 312:10

It is prohibited to walk four amohs¹²This law applies to males only. According to Mishnah Berurah, in our times, you are not even permitted to sit in the house with an uncovered head. For the equivalents of amoh, see glossary. or to utter sacred words with an uncovered head. Also small children must be trained to cover their heads in order to encourage them to have reverence for God, as it is said of Rav Nachman bar Yitzchak: The mother of Rav Nachman bar Yitzchak was told by Coldai, ("Coldai" referring to astrologers), "Your son will be a thief." [Upon hearing this] she did not permit him to uncover his head. She said to him, "Cover your head in order that there be upon you the reverence of Heaven. (Maseches Shabbos, 156 b.)