Musnad Ahmad 799:189

Hushaym narrated to us, Mansur and Yunus informed us, from al-Hasan, from Samurah ibn Jundub, "that when he used to pray with them, he would remain silent for two silences: when he began the prayer, and when he said, '{Nor of those who go astray}' [Al-Fatihah: 7], he would also remain silent for a brief moment." They disapproved of that from him, so he wrote to Ubayy ibn Ka'b. Ubayy wrote back to them that the matter was as Samurah had done.