The Prophet ﷺ said, "The month is like this." Shuba described Jabala's gesture, who in turn described Ibn Umar's gesture, indicating twenty-nine. He showed this by displaying the fingers of both hands twice, and on the third time he held back one finger.
أخبرنا محمد بن عبد الأعلى قال حدثنا خالد قال حدثنا شعبة عن جبلة بن سحيم
عن النبي ﷺ قال الشهر هكذا ووصف شعبة عن صفة جبلة عن صفة ابن عمر أنه تسع وعشرون فيما حكى من صنيعه مرتين بأصابع يديه ونقص في الثالثة إصبعا من أصابع يديه
Sourced by Bukhari, in the Book of Fasting, chapter titled "The statement of the Prophet ﷺ, 'When you see the crescent moon, fast, and when you see it, break your fast,'" hadith no. 1908, and Muslim, in the Book of Fasting, chapter on the obligation of fasting Ramadan upon sighting the crescent moon, etc., hadith no. 13/1080, from the hadith of Shubah, with the same chain. It is also found in Kubra, hadith no. 2452.