As part of John Mueller’s #AskGoogleWebmasters series, he was asked the question as to whether linking out to other websites helps or hinders SEO:
John answered that whilst linking out to others can be helpful to users find new sources or better understand your point.
He did however warn against linking out to others for the purpose of advertising or some kind of link exchange agreement. He also added a note of caution of linking out to any sites that are linked to in the comments of your post:
John recommend for these kind of links using the rel nofollow link attribute.
He also said that in most cases, if you’re linking out to others from your page in a natural way to sites that offer value and more context, then there’s nothing to worry about.
To hear exactly what John had to say, you can watch the full 1 minute 57 second video here.