Amar Attaher, 34, was hit by several bullets as he came out of a mosque, the sources said.
After the killing, Hamas officials using loud hailers called on businesses in Nablus to stage a strike Thursday in protest.
Both the West Bank and the Gaza Strip have in recent months seen violence between members of the ruling Hamas, which has been in government since March after winning Palestinian legislative elections in January, and the Fatah movement of president Mahmud Abbas.

Copyright AFP (Agence France-Presse), 2006