It is narrated from Ibn Umar (may Allah be pleased with them both) that when he prayed behind the Imam in Mina, he would perform four (obligatory) units, and when he prayed by himself, he would perform two units.
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 358
Hadith Takhrijأخرجه البيهقي في معرفة السنن والآثار: 1596 - ومالك في الموطأ: الصلاة، باب صلاة المسافر إذا كان إمامًا وراء الإمام.