It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Do not begin (fasting) ahead of the month until you see the new moon, or complete the count (of Sha'ban) before it."
Imam al-Tabarani (may Allah have mercy on him) said: "No one narrated it from Sufyan (may Allah have mercy on him) except al-Nu'man bin Abd al-Salam (may Allah have mercy on him)."