It is narrated from Hazrat Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: There is no reality in contagiousness of disease and evil omens, but I like optimism. I asked, "What is meant by optimism?" He replied, "A good word."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 13949