Al-Matalib al-'Aliyah 601 — Voluntary prayer on a mount

It is narrated from Umar bin Abdullah bin Urwah bin al-Zubayr that he heard Abdullah saying: "I arrived in Syria with Imam al-Zuhri on the return from the Battle of Yarmuk, and I saw him performing the (voluntary) prayer on his mount, whichever direction his mount was facing."
The chain of this report is jayyid (good), and it indicates the legal concession that it is permissible for a traveller to perform voluntary prayer on his mount, whether the mount faces the qiblah or any other direction.