"The Messenger of Allah vowed to stay away from his wives for a month, so he stayed in a loft for twenty-nine days. They said: 'O Messenger of Allah, your vow was for a month,' so he said: 'The month is twenty-nine (days).'"
Jami` at-Tirmidhi Book 8, Hadith 9
← Previous Hadith "What I fasted with the Prophet that was twenty-nine (days), was more than what we fasted that was thirty." (He said): There are narrations on this topic from `Umar, Abu Hurairah, `A'isha, Sa`d bin Abi Waqqas, Ibn `Abbas, Ibn `Umar, Anas, Jabir, Umm Salamah, Abu Bakrah, that the Prophet (ﷺ) said: "The month is twenty nine (days)."

Jami` at-Tirmidhi Book 8, Hadith 8 Next Hadith → "A Bedouin came to the Prophet and said: 'I have seen the crescent.' So he said: 'Do you testify that none has the right to be worshipped but Allah? Do you testify that Muhammad is the Messenger of Allah?' He said: 'Yes.' So he said: 'O Bilal! Announce to the people that they should fast tomorrow.'"

Jami` at-Tirmidhi Book 8, Hadith 10