Musnad Ahmad 32:628

Aswad ibn Amir narrated to us, Hammad ibn Salamah narrated to us, from Thumamah, from Anas, that the Prophet, peace and blessings of Allah be upon him, went out to them in Ramadan and made [the prayer] light for them. Then he entered and made [the prayer] long. Then he went out and made [the prayer] light for them. Then he entered and made [the prayer] long. When morning came, we said, "O Prophet of Allah, we sat [waiting] last night, and you came out to us and made it light, then you entered and made it long." He said, "I did that for your sake."