There have to be two witnesses for the sighting of the moon of shawwaal. There is a consensus among the four schools of thought, Hanafi, Maaliki, Shaafi'i, and Hanbali. The majority of scholars of Islaam hold this opinion. Evidence from the Sunnah It was narrated that: 'Abdur-Rahman bin Zaid bin Al-Khattab addressed the people on the day concerning which there was doubt (as to whether the month had begun) and said: "I sat with the Companions of the Messenger of Allah and asked them, and they a narrated that the Messenger of Allah said: 'Fast when you see it and stop fasting when you see it, and perform the rites on that basis. If it is obscured, then complete thirty days, and if two witnesses testify then fast and stop fasting ." [Sunan al-Nasaai'i Hadith No. 2116]
Quran and Sunnah
As understood by the Salaf