Musnad Ahmad 32:65

Muhammad ibn Abi ‘Adi narrated to us, from Humayd, from Anas, that the Prophet, peace and blessings of Allah be upon him, was praying one night in his chamber. Some people came and prayed with his prayer, so he shortened it, then entered the house, then came out, and returned repeatedly, all the while praying. When morning came, they said: "O Messenger of Allah, you prayed while we wished that you would lengthen your prayer." He said: "I knew of your presence, and I did that intentionally."