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). (Using translation from Abū Dāʾūd 2326)
عَنْ رَسُولِ اللهِ ﷺ قَالَ «لَا تَقَدَّمُوا الشَّهْرَ حَتَّى تَرَوُا الْهِلَالَ أَوْ تُكْمِلُوا الْعِدَّةَ ثُمَّ صُومُوا حَتَّى تَرَوُا الْهِلَالَ أَوْ تُكْمِلُوا الْعِدَّةَ قَبْلَهُ»