Isa bin Hafs reported,

I was with Ibn Umar on a journey. He prayed Dhuhr and Asr as two units each, then withdrew to a small carpet of his. He saw some people performing voluntary prayers and asked, "What are these people doing?"I replied, "They are praying voluntary prayers."He said, "Had I been one to pray before or after the obligatory prayer, I would have completed it in full. I accompanied the Messenger of Allah ﷺ, and he would not exceed two units while traveling, and Abu Bakr did the same until he passed away, as did Umar and Uthman ."

أخبرني نوح بن حبيب قال حدثنا يحيى بن سعيد قال

حدثنا عيسى بن حفص بن عاصم قال حدثني أبي قال

كنت مع ابن عمر في سفر فصلى الظهر والعصر ركعتين ثم انصرف إلى طنفسة له فرأى قوما يسبحون قال ما يصنع هؤلاء قلت يسبحون قال لو كنت مصليا قبلها أو بعدها لأتممتها صحبت رسول الله ﷺ فكان لا يزيد في السفر على الركعتين وأبا بكر حتى قبض وعمر وعثمان رضي الله عنهم كذلك