Muhammad ibn al-Hasan al-Saffar, from Muhammad ibn Isa, from Sulayman ibn Hafs al-Marwazi, who said: The jurist, Imam al-Hadi عليه السلام, peace be upon him, said: “Shortening the prayer is required at the distance of two barids, which is twenty-four miles, or at the distance of one barid when going and returning. One barid is six miles, and that is two farsakhs. Therefore, shortening the prayer applies at four farsakhs. So when a man leaves his house intending to travel twelve miles, which is four farsakhs, and he reaches two farsakhs while still intending to return another two farsakhs, he shortens his prayer. But if he turns back from his original intention when he reaches two farsakhs, and wishes to remain there, then he must perform the prayer in full. And if he had already shortened his prayer, then turned back from his intention, he must repeat the prayer.”