This reference is found in the “death and legacy” section Ashoka entry in Wikipedia
this entry states:
In his old age, he seemed to have come under the spell of his youngest wife Tishyaraksha. It is said that she had got his son Kunala, the regent in Takshashila, blinded by a wily stratagem. But the official executioners spared Kunala and he became a wandering singer accompanied by his favourite wife Kanchanmala. In Pataliputra, Ashoka hears Kunala’s song, and realizes that Kunala’s misfortune may have been a punishment for some past sin of the emperor himself and condemns Tishyaraksha to death, restoring Kunala to the court. Kunala was succeeded by his son, Samprati, but his rule did not last long after Ashoka’s death.