al-Husain bin Dhakwan al-Mua'lam الحسين بن ذكوان المعلم المكتب · Fasting (Kitab Al-Siyam)

The Prophet (ﷺ) said: Do not fast (for Ramadan) before the coming of the month until you sight the moon or complete the number (of thirty days); then fast until you sight the moon or complete the number (of thirty days).

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.