من لا يحضره الفقيه, volume 1, page 454

Hadith.1317 - Imam Jafar ibn Muhammad Al-Sadiq (as) was asked: "Why is the Maghrib prayer three rak'ahs with an additional four rak'ahs (supererogatory) afterward, and why is there no shortening of it during residence or travel?" Imam (as) said: "Indeed, Allah (swt), Blessed and Exalted, revealed to His Prophet (sw) every prayer as two rak'ahs. The Messenger of Allah (sw) added two rak'ahs to each prayer in residence and shortened them during travel, except for the Maghrib and Fajr (morning) prayers. When the Prophet (sw) performed the Maghrib prayer, he was informed of the birth of Sayyida Fatimah Al-Zahra {S.A}, so he added a rak'ah out of gratitude to Allah (swt), Mighty and Majestic. When Hasan (as) was born, he added two more rak'ahs out of gratitude to Allah (swt), and when Hussain (as) was born, he added two more rak'ahs out of gratitude to Allah (swt). He then said: 'For a male, the share is twice that of a female,' and thus he left the prayer in its current form during residence and travel."

Translation: Bab Ul Qaim Publications — attributed by Thaqalayn·The Arabic above is the original.

Hadith.1318 - Al-Fadl ibn Shadhan al-Naysaburi, may Allah (swt) have mercy on him, mentioned among the reasons he heard from al-Ridha (as) regarding why prayers are shortened during travel. He (Imam (as)) said: "The initially prescribed obligatory prayer was ten rak'ahs. The seven additional rak'ahs were later added, and Allah (swt), the Blessed and Exalted, lightened this addition for the servant due to the burden, fatigue, and preoccupation of travel. This relief was granted so that the servant would not become distracted from the essential needs of their livelihood during travel. However, the Maghrib prayer is not shortened because it was originally set as a shortened prayer. The obligation to shorten applies to eight farsakhs (a day's journey) and no less. The reason for this is that eight farsakhs is a day’s journey for the common people, caravans, and baggage carriers, necessitating shortening for such a journey. If the obligation were not established for a day’s journey, it would not be required for a journey of a thousand years, as every day after this day is similar to it ; if it were not obligatory for this day, it would not be obligatory for its equivalent. Voluntary prayers during the day are omitted while traveling, but not voluntary prayers at night, because every prayer that is not shortened also maintains its voluntary aspect. Thus, the Maghrib prayer is not shortened, so neither are the voluntary prayers that follow it. Similarly, the Fajr prayer is not shortened, so neither are the voluntary prayers preceding it. The Isha prayer (Atamah) is considered shortened, but its two rak'ahs are not omitted because they are not part of the fifty obligatory rak'ahs; rather, they are additional voluntary rak'ahs to complete the fifty. These extra rak'ahs serve as compensation, with two voluntary rak'ahs taking the place of every rak'ah of obligation. The traveler and the sick are permitted to perform the night prayer at the beginning of the night due to their preoccupation, weakness, or to ensure they fulfil their prayers and gain rest or focus on their travel."

Translation: Bab Ul Qaim Publications — attributed by Thaqalayn·The Arabic above is the original.