Al-Rabi' bin Sulayman (may Allah have mercy on him) says that Imam al-Shafi'i (may Allah have mercy on him) informed us: "Whoever deliberately vomits, making up the fast is obligatory on him; and whoever is overcome by vomiting, there is no making up due from him." Malik (may Allah have mercy on him) informed us of this.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8025