Musnad Ahmad 32:1122

Yazid narrated to us, Humaid informed us, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) was praying one night in his chamber. Some of his companions came and prayed with his prayer, so he shortened it. Then he entered the house, then he came out, and he did that repeatedly; each time he would pray and depart. When morning came, they said: "O Messenger of Allah, we prayed with you last night, and we would have liked for you to lengthen your prayer." He said: "I knew of your presence, and I did that intentionally."