Musnad Ahmad 32:2158

Bahz narrated to us, Hammad narrated to us, he said: Thumamah ibn Abdullah ibn Anas informed us, from Anas ibn Malik, that the Messenger of Allah (peace and blessings of Allah be upon him) had his companions come to him one night. He went out and prayed with them, and he made it light. Then he entered his house and made it long. Then he went out and prayed with them, and he made it light. Then he entered his house and made it long. When morning came, they said: "O Messenger of Allah, you prayed and you would make it long when you entered, and make it light when you went out." He said: "It was for your sake that I did what I did."