Musnad Ahmad

It is narrated from Hazrat Naeem bin Nahham (may Allah be pleased with him) that once, on a very cold night, the muezzin began the call to prayer. I started wishing in my blanket that he would announce, "Pray in your tents." When he reached "Hayya 'ala al-Falah," he made the announcement that there is no sin upon the one who prays in his own tent.