The Nāṣirī Generations 13: IV. TAHIR, SON OF 'ABD-ULLAH.

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When the account of the decease of Abd-ullah reached the Khalifah, Al-Wasik, he despatched, from the Dar-ul-Khilafat of Baghdad, letters patent and a standard, confirming him as his father’s successor.
His brothers solicited from Tahir the grant of the province of Khurasan, and its government; and he bestowed on his brother, Amir Mus’ab, the government of Nishapur. The Khalifah, Al-Wasik, died in the month Zu-l-Hijjah, 232 H., and Al-Mutawakkil assumed the Khilafat. He confirmed Tahir in the government of ’Ajam. After a period of fourteen years and nine months, at which time the Khalifah, Al-Mutawakkil, was martyred by the Turks, he was succeeded by Al-Mustansir.
Six months subsequently to that event, in the year 248 H., Al-Musta’in succeeded him. He sent letters patent and a standard, and confirmed Tahir, son of ’Abd-ullah, in his government, as before; and, in that same year, Amir Tahir died, having previously nominated his son Muhammad as his successor over Khurasan.