The Twitter API doesn't offer that option, so neither does LINQ to Twitter. That said, here's what you can do:
- Perform your search query as normal. It will contain retweets.
Perform a LINQ to Objects query on the results using a where clause that sets the condition of RetweetedStatus.StatusID == 0, like this:
var nonRetweetedStatuses = (from tweet in searchResponse.Statuses where tweet.RetweetedStatus.StatusID == 0 select tweet) .ToList();
The reason why I suggested using the where clause like this is because LINQ to Twitter instantiates a Status for RetweetedStatus, regardless of whether it exists or not. However, the contents of that Status are the default values of each properties type. Since StatusID will never be 0 for a valid retweet, this works. You could also filter on another field like Text == null and it would still work.