Abu Muhammad Musa ibn Mahdi al-Hadi (Arabic: أبو محمد موسى بن المهدي الهادي‎‎) (born: 147 AH [764 AD]; died: 170 AH [786 AD]) was the fourth Abbasid caliph who succeeded his father Al-Mahdi and ruled from 169 AH (785 AD) until his death in 170 AH (786 AD).

Al-Hadi was the eldest son of Al-Mahdi and Al-Khayzuran and like his father he was very open to the people of his empire and allowed citizens to visit him in the palace at Baghdad to address him. As such, he was considered an "enlightened ruler", and continued the progressive moves of his Abbasid predecessors.

His short rule was wrought with numerous military conflicts. The revolt of Husayn ibn Ali ibn Hasan broke out when Husayn declared himself caliph in Medina. Al-Hadi crushed the rebellion and killed Husayn and many of his followers, but Idris bin Abdallah, a cousin of Husayn, escaped and aided by Wadih, Egyptian postal manager, reached Morocco where he founded the Idrisi state. Al-Hadi also crushed a Kharijite rebellion as well as faced a Byzantine invasion. However, the Byzantines were turned back, and the Abbasid armies actually seized some territory from them.

Al-Hadi died in 786. al-Tabari notes varying accounts of this death, e.g. an abdominal ulcer or assassination prompted by al-Hadi's own step-mother. Al-Tabari (v. 30 p. 42f) notes al-Hadi's assertion of independence from his mother, his forbidding her further involvement in public affairs and his threatening Harun's succession. al-Tabari says others refer to al-Hadi's overtures to Harun. One account al-Tabari cites has al-Hadi attempting to poison his mother:

...
Wikipedia