פירוש על ברכות 59:7

THE HALAKHA IS IN ACCORDANCE WITH REBBE. Our Mishna mentions only the opinion of the first Tano that one who is riding on a donkey should descend from the donkey to pray, if he has someone who can watch the donkey. The Braiso introduces a dissenting opinion. Rebbe holds that even if one has a guard available, he need not descend from the donkey. The Gemara concludes that the halochoh is like Rebbe. That he need not dismount from the donkey, even when there is no danger. And even as the donkey is walking one may pray and he need not turn his face toward Yerusholayim.
Rebbe’s reason for not requiring that one descend from the donkey is because he will not be able to concentrate properly since he feels that he is losing time while praying. If we required that he stop moving or face Yerusholayim as he prays, that too would cost him time and prevent him from concentrating properly. Therefore, Tosfos concludes that since the halochoho is like Rebbe, not only does one not have to descend from the donkey, but he may continue praying as the donkey walks and he need not face Yerusholayim.

M.: Rab.