It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with both of them) that the Messenger of Allah (ﷺ) was performing prayer on his mount during the expedition of Banu Anmar, while his face was towards the east.
Hadith Referenceالسنن المأثورة / باب ما جاء في الصلاة على الراحلة / 77
Hadith Takhrijصحیح بخاری، المغازی، باب غزوة انمار ، رقم : 4140۔