It is narrated from Sayyiduna Malik bin al-Huwayrith (may Allah be pleased with him); he said: Two men came to the Prophet (peace and blessings be upon him) intending to travel, and the Prophet (peace and blessings be upon him) said: "When you two set out (on the journey), give the adhan, then the iqamah, and let the elder of you two lead the prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16688