Hafs bin Asim says that once we set out on a journey with Sayyiduna Ibn Umar (may Allah be pleased with them). We performed the obligatory prayer, and during this time, Sayyiduna Ibn Umar (may Allah be pleased with them) saw one of his sons performing voluntary prayers. Sayyiduna Ibn Umar (may Allah be pleased with them) said: I have performed prayers with the Prophet (peace and blessings be upon him) and with the three Caliphs during travel, but none of them would perform any prayer before or after the obligatory prayers. Sayyiduna Ibn Umar (may Allah be pleased with them) further said: If I were to perform voluntary prayers, then I would not have shortened my obligatory prayer (why would I shorten it?).