Narrated Abdullah ibn Umar:

The Prophet (ﷺ) said: The month consists of twenty-nine days, but do not fast till you sight it (the moon) and do not break your fast till you sight it. If the weather is cloudy, calculate it thirty days. When the twenty-ninth of Sha'ban came, Ibn Umar would send someone (who tried) to sight the moon for him. If it was sighted, then well and good; in case it was not sighted, and there was no cloud and dust before him (on the horizon), he would not keep fast the next day. If there appeared (on the horizon) before him cloud or dust, he would fast the following day. Ibn Umar would end his fasting alone with the people, and did not follow this calculation.