عَنْ رِبْعِيِّ بْنِ حِرَاشٍ عَنْ بَعْضِ أَصْحَابِ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ: ((لَا تَقَدَّمُوا الشَّهْرَ حَتَّى تُكْمِلُوا الْعِدَّةَ أَوْ تَرَوْا الْهِلَالَ وَصُومُوا وَلَا تُفْطِرُوا حَتَّى تُكْمِلُوا الْعِدَّةَ أَوْ تَرَوْا الْهِلَالَ))
A companion narrates that the Messenger of Allah, peace and blessings be upon him, said: Do not begin fasting before the month starts; rather, fast when the count of the previous month is completed or the new moon is sighted. Then continue fasting until the count of Ramadan is completed or the new moon is sighted.