It was narrated on the authority of Ibn ‘Umar, may Allah be pleased with him, that the Prophet, , said: "The month is thus and thus and thus (meaning thirty days).”Then, he said:“It may be thus and thus and thus (meaning twenty-nine days).”He meant that it is thirty days at one time and twenty-nine days in another. [Al-Bukhari and Muslim]
In another narration, the Prophet, , said: "We are an unlettered nation: we neither write nor calculate. The month is like this and this (i.e. sometimes twenty-nine days and sometimes thirty days)."[Al-Bukhari]
Ibn ‘Umar, may Allah be pleased with him, heard a man say, "Tonight is the half of the month.”Ibn ‘Umar asked,“How did you know that?”The man replied,“I heard the Prophet say:‘The month is like this and this’, and he opened and held his ten fingers twice and at the third time he opened all his fingers and held the thumb." [Muslim]
Sa‘d ibn Abi Waqqaas, may Allah be pleased with him, said, "The Prophet stuck his palms against one another and said:' The month is like this and this,' and he held one finger in the third time."[Muslim]
It was narrated on the authority of Ibn ‘Abbaas, may Allah be pleased with him, that the Prophet, , said: "Jibreel (Gabriel), may Allah exalt his mention, came and told me that the month is twenty-nine days." [An-Nasaa'i] [Al-Albaani: Saheeh]
It was narrated on the authority of Ibn Mas‘ood, may Allah be pleased with him, that he said, "We fasted twenty-nine days along with the Prophet more often than we fasted thirty days." [Abu Daawood, At-Tirmithi, Al-Bayhaqi and Ahmad] [Al-Albaani: Saheeh]
At-Tirmithi said, "For In this topic, it was narrated on the authority of ‘Umar, Abu Hurayrah, ‘Aa’ishah, Sa‘d ibn Abi Waqqaas, Ibn ‘Abbaas, Anas, Jaabir, Umm Salamah and Abu Bakrah that the Prophet said: 'The month is twenty-nine days.'"
Benefits and rulings:
First: The lunar month, which is the calendar for righteous deeds, is sometimes thirty days and sometimes twenty-nine days.
Second: The Muslim's reward is full even if the month is twenty-nine days. Ibn Mas‘ood, may Allah be pleased with him, said that they fasted twenty-nine days along with the Prophet, , more often than they fasted thirty days.
Third: These Hadeeths answer the people of astronomical calculations and astronomers, confirming that the Sharee‘ah rulings, such as fasting, breaking the fast and Hajj, are based on moon sighting, not on astronomical calculations.
Fourth: The permissibility of using gestures, which are means of explanation and teaching. [Fat-h Al-Baari]
Fifth: The lunar month could be twenty-nine days for two, three or four months consecutively, but not more than for four. [Sharh An-Nawawi ‘Ala Muslim]
Sixth: This Ummah (nation) is described as "unlettered", because writing was so rare among its members and their Prophet, , was an unlettered man. Allah The Almighty Says (what means):
{It is He who has sent among the unlettered a Messenger from themselves.} [Quran 62:2]
{And you did not recite before it any scripture, nor did you inscribe one with your right hand. Otherwise the falsifiers would have had [cause for] doubt.} [Quran 29:48]
It is the favor of Allah The Almighty that He blessed us with this great religion, because Muslims did not take their Book from others, but rather it came to them as a direct revelation from Allah The Almighty to His Messenger, . [‘Umdat Al-Qaari]
Seventh: This Ummah is in no need of any book or astronomical calculations in their acts of worship or determining their timing. Allah The All-Wise made them contingent on visual sighting, which is easier for those who are competent for religious assignment. [Tafseer ibn Katheer]
Eighth: We are not obliged to learn specific calculations or writing to know the times of our fasting or any act of worship; the times are connected with clear signs and things that people, mathematicians and otherwise, can equally recognize.
Ninth: For a Muslim who made an oath or a vow to fast a specific month, such as Rajab or Sha‘baan, and the days of fasting were twenty-nine, his oath would be fulfilled.
Tenth: If a Muslim made an oath or a vow to fast any month without specifying it, it would be enough for him to fast twenty-nine days, because the month is twenty-nine days. This is the preponderant opinion – Allah willing.
Eleventh: The doubtful day (e.g., the 30th of Sha‘baan) is considered one of the days of Sha‘baan, and it is not permissible to fast it as one of the days of Ramadan. That is because the Prophet, , made fasting Ramadan contingent on seeing the moon.
Twelfth: The Hadeeths signify that there is no harm on using modern instruments, such as observatories, telescopes and the like, in determining the location of the crescent to facilitate its sighting. They do not belong to the category of calculations which are nullified by the Hadeeths, as what is considered here is seeing with the eyes.