Whoever fasts the day of doubt has disobeyed Abu al-Qasim (peace and blessings be upon him)

Silah bin Zufar (may Allah have mercy on him) narrates: We were with Sayyiduna Ammar bin Yasir (may Allah be pleased with him), and he ordered a roasted goat to be brought and said: "Eat." Some of the people moved back and said that they were fasting. At this Sayyiduna Ammar (may Allah be pleased with him) said: "Whoever fasts on the day of doubt has certainly disobeyed Abu al-Qasim (peace and blessings be upon him)."
This hadith is sahih on the condition of the two Shaykhs, but the two Shaykhs did not record it.

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that a Bedouin came to the Prophet (peace and blessings be upon him) and said that he had seen the new moon, that is, the moon of Ramadan. He (peace and blessings be upon him) asked: "Do you bear witness that there is no god but Allah?" He said: Yes. He (peace and blessings be upon him) asked: "Do you bear witness that Muhammad is the Messenger of Allah?" He said: Yes. He (peace and blessings be upon him) said: "O Bilal! Announce to the people that they should fast tomorrow."
Imam Sufyan al-Thawri and Hammad bin Salamah have corroborated it from Simak bin Harb.

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that on the night of the new moon of Ramadan a Bedouin came and said: O Messenger of Allah! I have seen the new moon. He (peace and blessings be upon him) asked: "Do you bear witness that there is no god but Allah and that Muhammad is the Messenger of Allah?" He said: Yes. He (peace and blessings be upon him) said: "Then call out among the people that they should fast."
Al-Fadl bin Musa has likewise narrated it from Sufyan al-Thawri.

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that on the night of the new moon of Ramadan a Bedouin came and said: O Messenger of Allah! I have seen the new moon. He (peace and blessings be upon him) said: "Do you bear witness that there is no god but Allah and that I am the Messenger of Allah?" He said: Yes. He (peace and blessings be upon him) said: "Then announce that the people should fast."

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the people were in doubt about the new moon of Ramadan, and they intended (since it had not been confirmed) neither to perform the night prayer nor to fast. Meanwhile, a Bedouin came from the area of al-Harrah and testified that he had seen the new moon. So the Prophet (peace and blessings be upon him) ordered Sayyiduna Bilal (may Allah be pleased with him), and he called out among the people that they should both perform the night prayer and fast.
Imam Bukhari used the hadiths of Ikrimah as evidence, and Imam Muslim used the hadiths of Simak bin Harb and Hammad bin Salamah as evidence; this hadith is sahih, but the two Shaykhs did not record it.

Simak (may Allah have mercy on him) narrates: On the day of doubt about Ramadan I went to Ikrimah (may Allah have mercy on him), and he was eating. He said: "Come near and eat." I said: "I am fasting." He said: "By Allah! You will have to come near and eat." I said: "Then narrate a hadith to me." He said: "Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated to me that the Messenger of Allah (peace and blessings be upon him) said: 'Do not go ahead to receive the month in advance; rather, fast upon sighting the moon and break the fast upon sighting the moon. Then if a cloud or dust comes between you and the moon, complete a count of thirty days.'"
The chain of this hadith is sahih, but the two Shaykhs did not record it with these words.