There's only one reference to Krishna in the Ramayana. In this chapter of the Uttara Kanda, Rama asks Lakshmana to find out whether anyone is waiting to see him, because Rama was distraught over Sita's exile and thus wasn't seeing any subjects for the past few days. Rama tells Lakshmana that it is very important for a king to see his subjects promptly. He cites a story about the ancient king Nriga, who was cursed to turn into a lizard by two Brahmanas who were waiting to see him. Here is what the Brahmanas said:
Since thou hast not granted us an interview though we have been waiting here for a long time, thou shalt, by our curse, remain invisible, being a lizard... When Vishnu assuming a human person shall be born in the Vasudeva family of Yaduvas and enhance their glory he shall liberate thee from the curse. Immediately befere the advent of the Kali Yuga the Rishi Nara Narayana gifted with great prowess shall descend upon earth to relieve her of the burden.
So that's a prophecy of the birth of Krishna and Arjuna. And indeed Krishna did free Nriga from the curse, as described in this chapter of the Srimad Bhagavatam.
Note that the Uttara Kanda does have some interpolations, so it's possible that this chapter is a later addition after the time of Krishna. (In fact, some people even call the Uttara Kanda itself a later addition to the Ramayana, but I don't think that's correct.)
As far as other scriptures go, there are lots of incidents in the Puranas taking place long before even the time of Rama which prophesy the birth of Krishna, but I'm not aware of any scriptures older than the Ramayana prophesying him.