Hadith: Tiyarah (belief in evil omens) is Shirk (polytheism). Tiyarah is Shirk. Tiyarah is Shirk - three times." Every one of us experiences something of that, but Allah Almighty dispels it with reliance on Him

عَنْ عَبْدِ اللهِ بْنِ مَسعودٍ رضي الله عنه عن رَسولِ اللهِ صلى الله عليه وسلم قال:
«الطِّيَرَةُ شِرْكٌ، الطِّيَرَةُ شِرْكٌ، الطِّيَرَةُ شِرْكٌ، -ثلاثًا-»، وَمَا مِنَّا إِلَّا، وَلَكِنَّ اللهَ عَزَّ وَجَلَّ يُذْهِبُهُ بِالتَّوَكُّلِ.
[صحيح] - [رواه أبو داود والترمذي وابن ماجه وأحمد] - [سنن أبي داود: 3915]

‘Abdullah ibn Mas‘ūd (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said:
"Tiyarah (belief in evil omens) is Shirk (polytheism). Tiyarah is Shirk. Tiyarah is Shirk - three times." Every one of us experiences something of that, but Allah Almighty dispels it with reliance on Him.
Sahih/Authentic. - [Ibn Maajah]

Explanation

The Prophet (may Allah's peace and blessings be upon him) warned against Tiyarah, which is regarding something, audible or visible, as a bad omen whether it is from birds, animals, people with deformities, numbers, days, etc. He mentioned the "Tayr" (bird) in specific because it was commonly known during the pre-Islamic era of ignorance, as a bird would be urged to fly when someone intended to do something like travel, trade, or anything else. If the bird flew to the right, he would take this as a good omen and proceed with what he intended, whereas if the bird flew to the left, he would take this as a bad omen and refrain from doing what he intended. He informed us that it is Shirk. In fact, belief in bad omens is Shirk because none can bring goodness except Allah and none can ward off evil except Allah, alone with no partner.
Ibn Mas‘ūd (may Allah be pleased with him) mentioned that a Muslim may feel in his heart something of this pessimism; however, he must dispel it with reliance on Allah along with utilizing the available means.