Musnad Ahmad 30:845

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Dawud ibn Farahij, who said: I heard Abu Hurayrah say: The Prophet, peace and blessings of Allah be upon him, stayed away from his wives—Shu‘bah said: And I think he said: A month—so ‘Umar ibn al-Khattab came to him while he was in an upper room on a reed mat, and the mat had left an impression on his back. He said: "O Messenger of Allah, Chosroes drink from gold and silver, and you are like this?" The Prophet, peace and blessings of Allah be upon him, said: "They are those whose good things were hastened for them in the life of this world." Then the Prophet, peace and blessings of Allah be upon him, said: "The month is twenty-nine, like this and like this," and he folded his thumb on the third [time].