Vaishnava Samhita. Vitthalanatha Charitam. Vol III Book VII Chp 3 Shloka 7

कारयामास सप्रेम गीतानि व्रजभाषया |
कवीन्द्रैरष्टभिर्नित्यं श्रीनाथप्रियकिङ्करैः ||

Those eight bhagavatas, who were the devotees of Shrinatha and also great poets, wrote many songs and verses in the Vrija language.