Isa bin Hafs reported,

I was traveling with Ibn Umar when he prayed Dhuhr and Asr as two units each. He then walked over to a carpet, where he noticed some people performing voluntary prayers afterward.He asked, "What are these people doing?"I replied, "They are performing voluntary prayers."He said, "If I were going to pray before or after the obligatory prayer, I would simply complete it as a full four-unit prayer. I accompanied the Prophet ﷺ until his death, and he never prayed more than two units while traveling. I was with Abu Bakr until his death, and he did not exceed two units. Umar and Uthman did the same."

حدثنا يحيى

عن عيسى بن حفص حدثني أبي أنه قال

كنت مع ابن عمر في سفر فصلى الظهر والعصر ركعتين ركعتين ثم قام إلى طنفسة فرأى ناسا يسبحون بعدها فقال ما يصنع هؤلاء قلت يسبحون قال لو كنت مصليا قبلها أو بعدها لأتممتها صحبت النبي ﷺ حتى قبض فكان لا يزيد على ركعتين وأبا بكر رضي الله عنه حتى قبض فكان لا يزيد عليهما وعمر وعثمان كذلك