Narrated Hudhayfah: 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).
Related hadith on this topic
Explanation & Benefits
Hafiz Muhammad Ameen
In this narration, it is explicitly stated that one should refrain from fasting before the new moon is sighted. And this is what should be acted upon.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 2128