I was able to find the original paper on LSTM, I was not able to find the paper that introduced "vanilla" RNNs. Where can I find it?

I was able to find the original paper on LSTM, but I was not able to find the paper that introduced "vanilla" RNNs. Where can I find it?

I was able to find the original paper on LSTM (https://www.bioinf.jku.at/publications/older/2604.pdf). But I have had no luck with finding the original paper on RNN. Can someone please give me a link to the paper or point out what I am doing wrong?

Also, I am not sure if this is the right place to be asking this. If not, please let me know.

I was able to find the original paper on LSTM, I was not able to find the paper that introduced "vanilla" RNNs. Where can I find it?