Sahih al-Bukhari 1102

‘Abdullāh ibn ‘Umar (may Allah be pleased with him) mentioned that he accompanied the Prophet (may Allah's peace and blessings be upon him) on his journeys and that he accompanied Abu Bakr, ‘Umar, and ‘Uthmān (may Allah be pleased with them) in their travels as well, and all of them used to shorten the four-Rak‘ah prayers into two and never exceeded this number of Rak‘ahs.

In other words, they never offered four Rak‘ahs in the obligatory prayers and never performed the supererogatory prayers while on a journey. He mentioned Abu Bakr, ‘Umar, and ‘Uthmān to prove that this ruling was not abrogated – rather, it was applied after the death of the Prophet (may Allah's peace and blessings be upon him) – and that there is no preponderant objection to that ruling. It is permissible to not shorten the prayer on a journey; however, shortening is preferable,

for Allah, the Almighty, says: {When you are on a journey, it is no sin to shorten your prayers.} [Sūrat An-Nisā': 101] The negation of sin here implies that this is a concession and not the original ruling, which is praying the four Rak‘ahs. It is preferable for the traveler to act upon that concession and shorten the prayer in order to comply with the Sunnah of the Prophet (may Allah's peace and blessings be upon him) and avoid the relevant difference in opinion among the scholars, as some of them considered it obligatory, and because it is the opinion preferred by the majority of the scholars.