The reference that nearly all Global Warming debunkers cite when saying that it is the sun is Usoskin et al., 2005.

Interestingly, their study does show a very, very strong correlation between solar activity and global mean temperature (~0.7 to 0.8 at >95% confidence) over the last 1150 years. Read carefully though, because this very paper also points out that temperature and solar activity strongly diverge starting in the 1970's. In fact the last sentence (never cited by the debunkers) says that the observed warming since 1975 *must* have a non-solar source.