It is narrated from Nafi‘ (may Allah have mercy on him) that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) once camped at the place of "Dajnan" on a cold night, so he ordered the muezzin to say: «الصلاة فى الرحال» meaning, "Pray in your tents," then he explained that when the Noble Prophet (peace and blessings be upon him) was on a journey and the night was cold or it rained, he would order the muezzin to announce: «الصلاة فى الرحال» (In one narration, it is mentioned that he prayed inside the tent).
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1309