Maaseh Rav Siman 50
Can copy this into an AI, or connect the whole library to your assistant. For deeper learning, you can directly use the original seforim, sites, and apps.
נפילת אפים גם בשחרית בשמאל (וכ"כ בש"ע שלו סי' קל"א ס"ק ה') וא"א שומר ישראל כ"א בתענית צבור:
Sefaria Community Translation
Also at Shachrit one should use their left hand to fall on their face (during Tachanun). (As is written in Shulchan Aruch 131:5). And one does not say 'Shomer Yisrael' unless it is a communal fast day.