Za'ida bin Qadama al-Thaqifi زائدة بن قدامة الثقفي · 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) left Medina for Mecca till he reached 'Usfan, He then called for a vessel (of water). It was raised to his mouth to show it to the people, and that was in Ramadan. Ibn 'Abbas used to say: The Prophet (ﷺ) fasted and he broke his fast. He who likes may fast and he who likes may break.