The arrival of Ramadan is an event which can be confirmed by sighting the new moon, even if it is seen by only one just person, or by the passage of thirty days in the immediately preceding month of Sha`ban.
Ibn `Umar said: “The people were looking for the new moon and when I reported to the Messenger of Allah that I had seen it, he fasted and ordered the people to fast.” (Related Abu Dawud, Al-Hakim, and Ibn Hibban, who declared it to be Authentic.)
Abu Hurairah reported that the Prophet, peace and blessings be upon him, instructed: “Fast after you have seen it [the new crescent] and end the fast [at the end of the month] when you see it. If it is hidden from you, then wait until the thirty days of Sha`ban have passed.” (Related by Al-Bukhari and Muslim)
Commenting on these reports, At-Tirmidhi states: “Most knowledgeable people act in accordance with these reports. They say that it is correct to accept the evidence of one person to determine the beginning of the fast. This is the opinion of Ibnul-Mubarak, Ash-Shafi`i, and Ahmad. An-Nawawi says that it is the soundest opinion. Concerning the new moon of Shawwal [which signifies the end of the fast], it is confirmed by completing thirty days of Ramadan, and most jurists state that the new moon must have been reported by at least two just witnesses. However, Abu Thaur does not distinguish between the new moon of Shawwal and the new moon of Ramadan. In both cases, he accepts the evidence of only one just witness.”
Ibn Rushd comments that: “The opinion of Abu Bakr Ibnul-Mundhir, which is also that of Abu Thaur and, I suspect, that of the Dhahiri school of thought, is supported by the following argument given by Abu Bakr Al-Mundhiri: there is complete agreement that breaking the fast is obligatory, that abstaining from eating is based on one person’s report, and that the situation must be like that for the beginning of the month and for the ending of the month, as both of them are simply the signs that differentiate the time of fasting from the time of not fasting.
Ash-Shaukani observes: “If there is nothing authentic recorded that states that one may only accept two witnesses for the end of the month, then it is apparent, by analogy, that one witness is sufficient, as it is sufficient for the beginning of the month. Furthermore, worship based on the acceptance of one report points to the fact that such singular reports are accepted in every matter unless there is some evidence that specifies the peculiarity of specific cases, such as the number of witnesses concerning matters of wealth, and so on. Apparently this is the opinion of Abu Thaur.”