Musannaf Ibn Abi Shayba 3:1319

Ya'la narrated to us, from 'Abd al-Malik, from Sa'id ibn Jubayr, regarding a man who combines two surahs in one rak'ah: He said: "As for what is from the *Mi'in* (surahs with approximately one hundred verses), then bow with each surah. And as for what is from the *Mathani* and the *Mufassal*, then combine [them] if you wish."