Final answer:
Sunnis and Shias share the belief in the acceptance of the Hadith as an essential part of Islamic tradition, which complements the teachings of the Quran as guidance for Muslims.
Step-by-step explanation:
The belief that is shared by Sunnis and Shias is the acceptance of the Hadith. The Hadith are a collection of sayings, actions, and approvals of the Prophet Muhammad and play a crucial role in Islamic theology and law for both sects, acting as a source of religious guidance and practice. Both the Sunnis and the Shias consider the Hadith to have significant sanctity, although they may differ in which specific traditions they accept.
This common belief is grounded in the religious teachings that both denominations adhere to, despite their differences in leadership succession and other doctrinal matters. While Sunnis do not require that leadership of the community specifically comes from the descendants of Muhammad through Ali, both sects still uphold the sanctity and importance of the Prophet Muhammad's teachings as captured in the Hadith. The presence of theological differences should never overshadow the core principles that unite the Muslim ummah.