Musannaf Ibn Abi Shayba 3:6003

Abu Khalid al-Ahmar narrated to us, from al-Hajari, from Abu Iyad, from Abu Hurayrah, who said: They used to speak during the prayer, then [the verse] was revealed: "And when the Qur'an is recited, then listen to it and pay attention." They said: This is regarding the prayer.