Hadith 4:324
Azhar al-Samman narrated to us, from Ibn 'Awn, from al-Hasan and Ibn Sirin, they both said: If the fasting person induces vomiting, he does not break his fast. But if he vomits [involuntarily], he breaks his fast.
حدثنا أزهر السمان عن بن عون عن الحسن وبن سيرين قالا إذا أذرع الصائم القيء فلا يفطر وإذا تقيأ أفطر