Disable auto-tweeting new blogpost

Every time I add a new post on my self-hosted WordPress blog (martijnboersma.com), I automatically tweet about this. I’ve been trying to disable the function, but I cannot figure out how. I really hope I didn’t enable this function myself at some stage 😉

I do not have any auto-tweet plugin installed. The only social plugins I use are DiggDigg and the JetPack Twitter Widget. I have sifted through all the WordPress settings, as well as through the attached applications on the Twitter settings page, but without success.

Does anyone know how to disable automatically tweeting about new posts?