[INDOLOGY] Sumitrā

[Csaba Dezso]

Dear Colleagues,
It seems to be common knowledge when one does a search on the internet that Sumitrā, the wife of Daśaratha, was the daugther of the king of Kāśī / Magadha. However, I cannot find this information in Vālmīki’s Rāmāyaṇa (of course I might well have overlooked something). Do you perhaps know a textual source for this? 
Ānandarāmāyaṇa Sārakāṇḍa 1:70 says this, but perhaps there is an earlier source?
And of course there is Raghuvaṃśa 9:17: magadhakosalakekayaśāsināṃ duhitaro, where, curiously, Vallabhadeva reads malaya- instead of magadha-.
Many thanks for your help,
Csaba