Brachos 4:6-7

Brachos 4:6

One who is riding on a boat, in a wagon, or on a raft – and who therefore cannot stand to recite Shemoneh Esrei – must direct his intentions towards the Holy of Holies.

Brachos 4:7

Rabbi Elazar ben Azariah says that the musaf prayer is only recited with the congregation; the Sages say that it is recited both with and without a congregation. Rabbi Yehuda says that an individual is exempt from reciting musaf in a place where a congregation is saying it.