Wikipedia
Bay‘ah (, literally a "sale" or a "commercial transaction"), in Islamic terminology, is an oath of allegiance to a leader. It is known to have been practiced by the Islamic prophet Muhammad. Bay'ah is sometimes taken under a written pact given on behalf of the subjects by leading members of the tribe with the understanding that as long as the leader abides by certain requirements towards his people, they are to maintain their allegiance to him. Bay'ah is still practiced in countries such as Saudi Arabia and Sudan. In Morocco, bay'ah is one of the foundations of the monarchy. In many Islamic traditions, the meaning is to sell oneself to a spiritual master, pir or sheikh in exchange for the spiritual knowledge he gives.
Bai'at or Bay'ah (; literally a "sale" or a "transaction") is an Islamic practice of declaring on oath, one's allegiance to a particular leader. In an Islamic religious sense, this oath is the standard procedure of pledging allegiance to an Imam, a spiritual leader, or to sell oneself in exchange of the spiritual guidance given by an Imam. It is known to have been practiced by the Islamic prophet Muhammad and his successors (caliphs) after him and is a practice that also forms a notable feature within Ahmadiyya Islam. Following the death of a Caliph, the leader of the community, a pledge of allegiance is taken by the newly chosen Caliph from members of the community, based upon this prophetic model and the bay'ah is the standard procedure for entering the Ahmadiyya Muslim Community. Since 1993, a large Bay'ah ceremony of initiation for entering the Ahmadiyya Muslim Community, is also held on an international scale every year during the annual gathering of the community. The ceremony is led by the caliph at whose hand new converts take the oath of initiation, and old members of the community can reaffirm their oaths.