Musnad Ahmad 33:470

Hujayn and Yunus narrated to us, they said: Layth ibn Sa'd narrated to us, from Abu al-Zubayr, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) secluded himself from his wives for a month. Then he came out to us on the twenty-ninth. We said: "Today is only the twenty-ninth." He said: "The month is [like this]," and he clapped his hands three times, and held back one finger in the last [time]. Yunus said: "One finger."