Zurarah narrated from Imam Muhammad ibn Ali Al-Baqir عليه السلام that He said: "There are four categories of people who are required to offer the full (complete) prayer whether they are traveling or at home: the mule driver (al-Mukari), the camel driver (al-Karri), the shepherd, and al-Ashtaqan (those who constantly travel as part of their work). This is because it is their work (which involves constant travel)." It was also narrated: "The sailor (al-Mallah) and al-Ashtaqan (a type of courier or mail carrier) are also included."