The Buddha declared him foremost among those who could inspire speech in others. AN.i.25
The Theragāthā Thag.133–134 contains two verses spoken by him in praise of concentration of the mind.
The Rādha Saṃyutta SN.iii.188–201 contains a large number of suttas preached by the Buddha in answer to Rādha’s questions on various topics.
If the reference is to this same thera, Rādha was, for some time, the attendant of Sāriputta, and there is a verse in the Theragāthā Thag.993 spoken to him by Sāriputta, who was pleased with Rādha’s gentle manner.