Musnad Ahmad 938:7

'Affan narrated to us, Wuhayb narrated to us, Musa ibn 'Uqbah narrated to us, he said: I heard Abu al-Nadr narrating from Busr ibn Sa'id, from Zayd ibn Thabit: That the Prophet, peace and blessings of Allah be upon him, made a chamber in the mosque out of a reed mat. The Messenger of Allah, peace and blessings of Allah be upon him, prayed in it for several nights, until people gathered for him. Then they missed his voice, and they thought that he had slept, so some of them began to clear their throats so that he would come out to them. He said: "What I saw of your behavior continued with you until I feared that it would be made obligatory upon you. And if it were made obligatory upon you, you would not be able to perform it. So pray, O people, in your houses, for the best of a person's prayer is in his house, except for the obligatory prayer."