Musnad Abu Ya'la al-Mawsili 129:142

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, Humayd al-Tawil informed us, from Anas bin Malik, that the Messenger of Allah (peace and blessings of Allah be upon him) prayed one night in his chamber, and the people heard his voice. When the second night came, people came and prayed with his prayer, so the Messenger of Allah (peace and blessings of Allah be upon him) lightened it, then he departed. When they reached the morning, they said: "O Messenger of Allah, we prayed with you last night, and we would have liked for you to lengthen your recitation." He said: "I knew of your presence, and I did that intentionally."