Explanation of the hadith
It was the habit of the Prophet (may Allah's peace and blessings be upon him) to recite long Sūrahs in the Fajr prayer, short Sūrahs in the Maghrib prayer, and medium-length Sūrahs in the rest of the five prayers. However, he would sometimes change that habit for the sake of showing the permissibility or for other purposes. This applies to the Hadīth in hand, which mentions that the Prophet (may Allah's peace and blessings be upon him) recited Sūrat At-Tūr, which is a one of the longer Sūrahs of the "Mufassal" section of the Qur'an in the Maghrib prayer.