Prophet (SAW)'s Prayers Upon the thought of a bad omen

If for no reason the thought of a bad omen comes to mind, then recite:

اللَّهُمَّ لَا يَأْتِي بِالْحَسَنَاتِ إِلَّا أَنْتَ وَلَا يَذْهَبُ بِالسَّيِّئَاتِإِلَّا أَنْتَ وَلَا حَوْلَ وَلَا قُوَّةَ إِلَّا بِكَ

"O Allah, you bring into existence the good and only you can remove any bad condition, as the power to do good and prevent from evil is in Your control".