Some time ago I was approached by a fellow researcher that had an idea on an earlier work of mine. We started a collaboration and as a result we submitted a manuscript in a prestigious journal. Recently, I stumbled on a publication of my coauthor in which he had used almost verbatim a large part of our common submission (a page long). These parts include results which were entirely mine. I am really disappointed and I am not sure how to proceed with this.
The submitted manuscript is proof enough that you also have worked on the results he published alone. You have authorship rights over all the content of the manuscript, do not worry searching for proofs that you have worked on those particular lines.
You should contact your collaborator and ask for a retraction of the article. If he disagree with that, you should contact the editor and send proofs that the results were submitted before in another journal, which breaks one of the rules your collaborator accepted when submitting to this journal: "The manuscript/results shall not be/were not submitted anywhere else.".
This is very serious because if the editor handling your manuscript find the other publication with the same results he will reject the manuscript and, worse, your co-author could be accused of self-plagiarism.