Musannaf Ibn Abi Shayba 3:1318

Ya'la ibn 'Ubayd narrated to us, he said: 'Abd al-Malik ibn Abi Sulayman narrated to us, from 'Ata', regarding the man who prays the obligatory prayer and recites two surahs in one rak'ah, or one surah in two rak'ahs: He said: There is no harm in it.