Musnad Ahmad 799:4

Muhammad ibn Ja'far narrated to us, Sa'id narrated to us, from Qatadah, from al-Hasan, from Samurah ibn Jundub, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, had two silences in his prayer." Imran ibn Husayn said: "I do not memorize them from the Messenger of Allah, peace and blessings of Allah be upon him." So they wrote to Ubayy ibn Ka'b regarding that, asking him about it. Ubayy wrote back: "Indeed, Samurah has memorized."