´It was narrated from Ribii bin Hirash, from Hudhaifah bin Al-Yaman, that the Messenger of Allah said:` "Do not anticipate the month until you see the crescent before it, or you complete the number of days. Then fast until you see the new moon, or you complete the number of 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