Yahya related to me from Malik from Nafi that Abdullah ibn Umarused to say, "If some one makes himself vomit while he is fasting hehas to make up a day, but if he cannot help vomiting he does not haveto make up anything." (IT)
USC-MSA web (English) reference: Book 18, Hadith 47 (IT)
Arabic reference (IT): Book 18, Hadith 680 (IT)