Columbia subject headings:
Harvard subject headings:
Abū Bakr, Caliph, -634.
Caliphate--History
Islamic Empire--History--622-661.
Princeton subject headings:
Abū Bakr, Caliph, -634.
Caliphs--Islamic Empire--Biography.
Islamic Empire--History--622-661.
Muḥammad, Prophet, -632--Companions.