al-Musta'in

Ahmad ibn Muhammad (; 836 – 17 October 866), better known by his regnal name (, "He who looks for help to God") was the Abbasid Caliph from 862 to 866, during the "Anarchy at Samarra". After the death of previous Caliph, al-Muntasir (who had not appointed any successors), the Turkish military leaders held a council to select his successor. They were not willing to have al-Mu'tazz or his brothers; so they elected al-Musta'in, a grandson of al-Mu'tasim.