Musnad Abu Ya'la al-Mawsili 109:265

Kamil narrated to us, Layth narrated to us, he said: Abu al-Zubayr narrated to me, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) secluded himself from his wives for a month. He came out on the twenty-ninth night, so we said: "Only twenty-nine have passed." He said: "The month is like this," and he clapped three times, and he held back one finger in the last [time].