Observe fast on sighting it (the new moon) and break (fast) on sighting it (the new moon), but if the sky is cloudy for you, then complete the number (of thirty). (Using translation from Muslim 1081b)

قَالَ رَسُولُ اللهِ ﷺ صُومُوا لِرُؤْيَتِهِ وَأَفْطِرُوا لِرُؤْيَتِهِ فَإِنْ غُمَّ عَلَيْكُمْ فَصُومُوا ثَلَاثِينَ