Musnad Ahmad 1155:29

Yazid ibn Harun narrated to us, he said: Nafi‘ ibn ‘Umar—and he is al-Jumahi—informed us, from Ibn Abi Mulaykah, that one of the wives of the Prophet, may Allah bless him and grant him peace—and I do not know her to be other than Hafsah—was asked about the recitation of the Messenger of Allah, may Allah bless him and grant him peace. She said: "You would not be able to bear it." She said: "{All praise is due to Allah, Lord of the worlds, the Entirely Merciful, the Especially Merciful} [Al-Fatihah: 2-3]," meaning the measured recitation (tartil).