Musnad Ahmad 33:626

Musa and Hassan narrated to us—and the wording is Hassan’s—they said: Ibn Lahi’ah narrated to us, Abu al-Zubayr narrated to us, he said: I asked Jabir, "Did you hear the Prophet, peace and blessings of Allah be upon him, say: 'A man is in prayer as long as he is waiting for the prayer'?" He said: We waited for the Prophet, peace and blessings of Allah be upon him, one night for the night prayer (al-‘atamah), and he was delayed from us until it was nearly half the night—or it reached that—then the Prophet, peace and blessings of Allah be upon him, came and we prayed. Then he said: "Sit." He addressed us, and the Prophet, peace and blessings of Allah be upon him, said: "Indeed, the people have prayed and slept, and you have not ceased to be in prayer as long as you were waiting for the prayer."