Musnad Ahmad 28:468

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 spoke and mentioned the hadith. Then Abdullah ibn Amr ibn al-Aas 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. Some remained and some returned. Then the Messenger of Allah, peace and blessings of Allah be upon him, came before the people rose for the Isha prayer. He came while he was breathing heavily, raising his finger like this—and he counted 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 these servants of Mine; they have performed an obligatory duty and they are awaiting another.'"