Al-Fadl ibn al-‘Abbās (may Allah be pleased with him) said: "Tiyarah is what prompts you to do something or to refrain from doing it."
عن الفضل بن العباس -رضي الله عنهما "إنما الطِّيَرَةُ ما أَمْضَاكَ أو رَدَّكَ".
شرح الحديث :
The forbidden Tiyarah – which is regarded as polytheism – is what makes a person proceed with what he intended to do or turn back from it, based on the bad omen. If, for instance, someone intended to travel then refrained from it because of a bad omen, then he has committed polytheism, shunned reliance on Allah, and made himself vulnerable to fear. What is understood from this Hadīth is that bad omens do not harm a person if they do not affect his determination to do or not to do something.