Sunan an-Nasai Book 15, Hadith 1458

أَخْبَرَنِي نُوحُ بْنُ حَبِيبٍ، قَالَ حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا عِيسَى بْنُ حَفْصِ بْنِ عَاصِمٍ، قَالَ حَدَّثَنِي أَبِي قَالَ، كُنْتُ مَعَ ابْنِ عُمَرَ فِي سَفَرٍ فَصَلَّى الظُّهْرَ وَالْعَصْرَ رَكْعَتَيْنِ ثُمَّ انْصَرَفَ إِلَى طِنْفِسَةٍ لَهُ فَرَأَى قَوْمًا يُسَبِّحُونَ قَالَ مَا يَصْنَعُ هَؤُلاَءِ قُلْتُ يُسَبِّحُونَ ‏.‏ قَالَ لَوْ كُنْتُ مُصَلِّيًا قَبْلَهَا أَوْ بَعْدَهَا لأَتْمَمْتُهَا صَحِبْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم فَكَانَ لاَ يَزِيدُ فِي السَّفَرِ عَلَى الرَّكْعَتَيْنِ وَأَبَا بَكْرٍ حَتَّى قُبِضَ وَعُمَرَ وَعُثْمَانَ - رضى الله عنهم - كَذَلِكَ ‏.‏

Eisa bin Hafs bin 'Asim said:"My father told me: 'I was with Ibn Umar on a journey, and he prayed Zuhr and 'Asr with two rak'ahs each, then he went and sat on his carpet. He saw some people offering voluntary prayers and said: What are these people doing? I said: They are offering voluntary prayers. He said: If I had wanted to pray before and after (the obligatory prayer) I would have offered it in full. I accompanied the Messenger of Allah (ﷺ) and he did not pray more than two rak'ahs when traveling, and Abu Bakr (did likewise) until he died, as did 'Umar and 'Uthman, may Allah (SWT) be pleased with them all