Musannaf Ibn Abi Shaybah

Ibrahim narrates that Hazrat Utbah bin Farqad was hiding in the villages. People saw the moon at the end of the day and broke their fast. This news reached Hazrat Umar, so he wrote a letter in which he said: If the moon is seen at the beginning of the neck (early evening), then it is the moon of the previous day; you may break your fast upon it. But if the moon is seen at the end of the day, then it is the moon of the coming day; in this case, complete your fast.