Musnad Abu Ya'la al-Mawsili 129:38

Wahb ibn Baqiyyah narrated to us, Khalid informed us, from Humayd, from Anas, who said: The Prophet, may Allah bless him and grant him peace, was praying in his chamber one night, and some of his companions came and prayed with his prayer. He said: Then he entered the house, then he came out, and he returned repeatedly, all the while praying. When morning came, they said: "O Messenger of Allah, we prayed with you, and we would have liked for you to lengthen your prayer." He said: "I knew of your presence, and I did that intentionally."