You will want to edit this file: jetpack/modules/sharedaddy/sharing-sources.php
return '<div class="twitter_button"><iframe allowtransparency="true" frameborder="0" scrolling="no" src="http://platform.twitter.com/widgets/tweet_button.html?url=' . rawurlencode( apply_filters( 'sharing_permalink', get_permalink( $post->ID ), $post->ID, $this->id ) ) . '&counturl=' . rawurlencode( str_replace( 'https://', 'http://', get_permalink( $post->ID ) ) ) . '&count=horizontal&text=Via @USERNAME: ' . rawurlencode( apply_filters( 'sharing_post_title', $post->post_title, $post->ID, $this->id ) ) . ': " style="width:97px; height:20px;"></iframe></div>';';
The tweet would then Read Via Username: TITLE OF THE POST: <URL>
Hope this helps.