Columbia subject headings:
Caliphs.
Muḥammad, Prophet, -632--Companions--Biography.
ʻUmar ibn al-Khaṭṭāb, Caliph, -644.
Harvard subject headings:
Islamic Empire--Kings and rulers--Biography.
ʻUmar ibn al-Khaṭṭāb, Caliph, -644.
Princeton subject headings: