Sunan Ibn Majah, Chapter: The Book of the Prayer

"The Messenger of Allah (saw) led us for the Maghrib prayer. Then he did not come out until half the night had passed. Then he came out and led them in prayer, then he said: 'The people have prayed and gone to sleep, but you are still in a state of prayer so long as you are waiting for the (next) prayer. Were it not for the weak and the sick, I wanted to delay this prayer until the middle of the night.'" (Sahih)