Musnad Ahmad 27:1614

Sulayman ibn Hayyan narrated to us, from al-Hasan—meaning ibn Ubayd Allah—from Sa'd ibn Ubaydah, that he heard Ibn Umar [say to] a man who was saying: "The middle of the night [the fifteenth night]," he said: "And what makes you know that it is the middle? Say: fifteen." I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "The month is like this, and like this, and like this." And Abu Khalid clenched [his fingers] in the third [instance] to mean fifty.