Isa bin Talhah narrates: I accompanied Ibn Umar (may Allah be pleased with them both) on a journey; he would not pray more than two rak'ahs (in the obligatory prayer), while his sons and companions would stand up to offer voluntary prayers. I asked: Why do you not offer voluntary prayers? He said: "I only do as I saw the Prophet (peace and blessings be upon him) doing."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13200