The Prophet ﷺ said: if one has a sudden attack of vomiting while one is fasting, no atonement is required of him, but if he vomits intentionally he must make atonement.  

قَالَ رَسُولُ اللَّهِ ﷺ مَنْ ذَرَعَهُ قَىْءٌ وَهُوَ صَائِمٌ فَلَيْسَ عَلَيْهِ قَضَاءٌ وَإِنِ اسْتَقَاءَ فَلْيَقْضِ  

That Abu ad-Darda' narrated to him: The Messenger of Allah ﷺ vomited and broke his fast. Then I met Thawban, the client of the Messenger of Allah ﷺ, in the mosque in Damascus, I said (to him): Abu al-Darda has told me that the Messenger of Allah ﷺ vomited and broke his fast. He said: He spoke the truth ; and I poured out water for his ablution ﷺ.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَاءَ فَأَفْطَرَ فَلَقِيتُ ثَوْبَانَ مَوْلَى رَسُولِ اللَّهِ ﷺ فِي مَسْجِدِ دِمَشْقَ فَقُلْتُ إِنَّ أَبَا الدَّرْدَاءِ حَدَّثَنِي أَنَّ رَسُولَ اللَّهِ ﷺ قَاءَ فَأَفْطَرَ قَالَ صَدَقَ وَأَنَا صَبَبْتُ لَهُ وَضُوءَهُ ﷺ