According to Yakubi...

According to Yakubi, it was on the 27th of Ramadan, in the year 201 Hijrah that the Imam was officially designated as Mamun’s “heir- apparent,” (Wali’ ahd,) and the Imam’s name was included with his own on gold and silver coins.

The inscription on these coins is well worthy of notice: “The King of God and the faith, Al-Ma’mun, Amir and Khalifa of the faithful, and al-Riḍa, the Imam of the Muslims.” He then announced to those he had summoned, that he had carefully considered all the descendants of Abbas and also the descendants of ‘Ali, and that he had not found in his search anyone more worthy or fitter to be his successor than ‘Ali al-Riḍa.

He, therefore, took him by the hand and publicly acknowledged him as his successor (Kulayni). Once this story of succession was announced Ma’mun’s political ambition was achieved, as he has secured the help of the Persians fully behind him and he…