Al-Matalib al-'Aliyah 733 — Shortening the prayer while travelling

It is narrated from Ishaq that he said: I asked Abu Usamah: Did Sa'd bin Sa'id al-Ansari narrate this hadith to you, that he heard Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) say: "The Zuhr and Asr prayers were (originally) made obligatory as two rak'ahs each, so the people used to pray two rak'ahs after Zuhr and two rak'ahs after Asr. Then four rak'ahs of Zuhr and four of Asr were made obligatory upon them, so when these four rak'ahs were made obligatory, they stopped praying those (earlier extra rak'ahs); and the travel prayer was kept (in its original form, that is, two rak'ahs), while the prayer of the resident, that is, when one is at home, was fixed at four rak'ahs?" At this Abu Usamah affirmed it and said: "Yes (it is exactly so)."
This hadith is hasan (good).