Musnad Ahmad 33:412

Rawh narrated to us, Zakariya narrated to us, Abu al-Zubayr narrated to us, that he heard Jabir saying: The Messenger of Allah (peace and blessings of Allah be upon him) stayed away from his wives for a month. He would be in the upper room, and they would be in the lower room. Then the Prophet (peace and blessings of Allah be upon him) came down to them on the twenty-ninth night. A man said: "O Messenger of Allah, you stayed for twenty-nine nights." The Messenger of Allah (peace and blessings of Allah be upon him) said: "The month is like this and like this," gesturing with the fingers of his hands twice, and on the third time, he held back his thumb.