" صُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ حَالَ بَيْنَكُمْ وَبَيْنَهُ سَحَابٌ فَأَكْمِلُوا الْعِدَّةَ وَلاَ تَسْتَقْبِلُوا الشَّهْرَ اسْتِقْبَالاً "
Ibn 'Abbas narrated that the Messenger of Allah said: "Fast when you see it, and stop fasting when you see it, and if clouds prevent you from seeing it, then complete the number, and do not fast ahead of Ramadan."