Musannaf Ibn Abi Shayba 6:402

Abdullah ibn al-Mubarak narrated to us, from Hammam, from Qatadah, from al-Hasan, from Qays ibn Abbad, who said: The companions of Muhammad, may Allah bless him and grant him peace, used to consider it desirable to lower the voice at three times: at the time of fighting, at the time of the Quran, and at the time of the funerals.