Sahih Muslim Book 26, Hadith Number 5516.

Chapter : There is no infection, no evil omen, no Hama, no safar, no star promising rain, no ghoul and the ill should not go to those who are healthy.

Jabir b. ‘Abdullah reported Allah’s Apostle (may peace be upon him) as saying: There is no transitive disease, no safar, no ghoul. He (the narrator) said: I heard Abu Zubair say: Jabir explained for them the word safar. Abu Zubair said: safar means belly. It was said to Jabir: Why is it so? He said that it was held that safar implied the worms of the belly, but he gave no explanation of ghoul. Abu Zubair said: Ghoul is that which kills the travellers.