Hameed bin Abi Hameed حميد بن أبي حميد الطويل · The Book on Fasting

"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).'"

Anas bin Malik was asked about the Prophet's fasting and he said: "He would fast during a month until we thought that he did not want to abstain from fasting during any of it. And, he would not fast during a month until we thought that he did not want to fast during any of it. (There was no time) that I wanted to see if he was performing Salat during the night, except that I would see him praying,…

"The Prophet would perform I'tikaf during the last ten (nights) of Ramadan. One year he did not perform I'tikaf, so he performed I'tikaf for twenty (nights) in the following year."