The Messenger of Allah ﷺ said, "Do not begin the month until you see the crescent or complete the count of thirty days, then fast until you see the crescent or complete the count of thirty days."
حدثنا محمد بن الصباح البزاز حدثنا جرير بن عبد الحميد الضبي عن منصور بن المعتمر عن ربعي بن حراش
قال رسول الله ﷺ لا تقدموا الشهر حتى تروا الهلال أو تكملوا العدة ثم صوموا حتى تروا الهلال أو تكملوا العدة
[Its isnad is sahih] Nasai narrated it in "Fasting," the chapter on the differing reports from Mansur regarding Rubai's hadith on this matter, no. 2128, from the hadith of Jareer through this route, and Ibn Khuzayma graded it sahih, no. 1911, as did Ibn Hibban, no. 875.