Is it permissible to recite a portion of a long Surah after Al-Fatiha in prayer instead of a complete Surah?

It is permissible to recite part of a long surah (definition) during prayer, but it is better to recite a complete surah. This is what the Prophet, peace and blessings be upon him, generally did. It has been reported that he sometimes recited part of a surah, such as his recitation of a portion of Surat Al-Baqarah and Al 'Imran in the two rak'ahs of Fajr prayer. It has also been reported that Ibn Mas'ud combined surahs in a single rak'ah, which could have been in obligatory or voluntary prayers. The best approach is to recite a complete surah in each rak'ah, but if that is difficult, there is no harm in dividing a surah between the two rak'ahs.