It is narrated from Hazrat Abu Musa (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) once explained by gesture that in travel, one should perform prayer on the back of the animal in this manner.
Hadith Referenceمسند احمد / تتمہ مُسْنَدِ الْكُوفِيِّينَ / 19702
Hadith Gradingحکم دارالسلام:إسناده ضعيف، الضعف يونس بن الحارث