Radha Krishna Statue

Radha Krishna's first literary mention was found in King Hala's Prakrit text Gatha Saptasati which is composed of 700 verses and was written in the 1st century CE. Later, The popular Gita Govinda written by Jayadeva in 12th century CE widely depicted Radha and Krishna as a couple. According to scriptures like Brahma Vaivarta Purana and Garga Samhita, Radha-Krishna are the supreme deities. 

The other relevant texts mentioning Radha Krishna are Radhopnishad of Rig Veda, Radhatapani Upanishad of Atharva Veda, Shiva Puran , Brahmanda Puran, Skanda Puran, Padma Puran, Matsya Puran, Devi-Bhagavata Purana, Narada Pancharatra and Brahma samhita. Goddess Radha is also indirectly mentioned in Bhagavata Purana of Sukadeva Goswami along with Lord Krishna under many different names like "Aradhika" and "Gopi". 

Adi Shankracharya who happened way before Jayadeva also mentioned Goddess Radha in his work called "Achyuta Ashtakam" which is dedicated to Achyuta form of Lord Krishna.