Al-Matalib al-'Aliyah 728 — Shortening the prayer while travelling

A man narrates from Sayyiduna Abu Dharr (may Allah be pleased with him): I came to him at the place called al-Baldah (which is the area of Mina), and it was said to him that Sayyiduna Uthman (may Allah be pleased with him) had led four rak'ahs (here, despite being a traveller). At this he said: "I prayed with the Prophet (peace and blessings be upon him), Sayyiduna Abu Bakr and Sayyiduna Umar (may Allah be pleased with them both), and all of them led only two rak'ahs (in Mina)." Then (even though his own scholarly position was two rak'ahs) he stood up and himself prayed four rak'ahs.
Imam Ahmad bin Mani' recorded this narration in his Musnad; its chain contains an unnamed (unknown) narrator. However, the account of Sayyiduna Uthman (may Allah be pleased with him) completing the prayer in full, and of the Companions (may Allah be pleased with them) following him (for the sake of unity), is also reported in other reliable books of hadith.