Hadith: When the Messenger of Allah (may Allah's peace and blessings be upon him) was on a journey and wished to offer voluntary prayers, he made his she-camel face the Qiblah, said Takbīr, and then prayed towards whatever direction his mount would face

عن أَنَسِ بْنِ مَالِكٍ رضي الله عنه:

أَنَّ رَسُولَ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ كَانَ إِذَا سَافَرَ فَأَرَادَ أَنْ يَتَطَوَّعَ اسْتَقْبَلَ بِنَاقَتِهِ الْقِبْلَةَ فَكَبَّرَ، ثُمَّ صَلَّى حَيْثُ وَجَّهَهُ رِكَابُهُ.
[صحيح] - [رواه أبو داود]

Anas (may Allah be pleased with him) reported: When the Messenger of Allah (may Allah's peace and blessings be upon him) was on a journey and wished to offer voluntary prayers, he made his she-camel face the Qiblah, said Takbīr, and then prayed towards whatever direction his mount would face.
Sahih/Authentic. - [Abu Dawood]

Explanation

Whenever the Prophet (may Allah's peace and blessings be upon him) was on a journey and wanted to offer a voluntary prayer, he would face the Qiblah on his mount while reciting the Takbīr of Ihrām, and then he would pray in whatever direction he was heading to.