It is narrated from Sayyiduna Malik bin al-Huwayrith (may Allah be pleased with him): A cousin of mine and I came to the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "When you travel, give the adhan and the iqamah, and let the elder of you two lead you in prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1942