Musnad Ahmad 28:275

Hassan ibn Musa narrated to us, Hammad ibn Salamah narrated to us, from Ali ibn Zayd, from Mutarrif ibn Abdullah ibn al-Shikhkhir, that Nawf and Abdullah ibn Amr met. Nawf said—and he mentioned the hadith—then Abdullah ibn Amr ibn al-As said: "And I shall narrate to you from the Prophet, peace and blessings of Allah be upon him: We prayed with the Prophet, peace and blessings of Allah be upon him, one night, and some remained while some returned. The Messenger of Allah, peace and blessings of Allah be upon him, came before the people had gathered for the Isha prayer. He came while he was short of breath, raising his finger like this—and he tied twenty-nine—and he pointed with his index finger toward the sky, while he was saying: 'Rejoice, O assembly of Muslims! This is your Lord, Mighty and Majestic is He, who has opened a door from the doors of the sky, boasting of you to the angels. He says: "O My angels, look at My servants; they have performed a prescribed duty, and they are awaiting another."'"