Musnad Ahmad 27:313

Waki’ narrated to us, ‘Isa ibn Hafs ibn ‘Asim narrated to us, from his father, who said: We went out with Ibn ‘Umar and we prayed the obligatory prayer. He saw some of his children performing voluntary prayers, so Ibn ‘Umar said: "I prayed with the Prophet, may Allah bless him and grant him peace, and Abu Bakr, and ‘Umar, and ‘Uthman during travel, and they did not pray before it, nor after it." Ibn ‘Umar said: "And if I were to perform voluntary prayers, I would have completed [the obligatory prayer]."