Musnad Ahmad 1156:136

Yahya ibn Sa'id narrated to us, from al-A'mash, who said: Shaqiq narrated to me, and Ibn Numayr said: al-A'mash narrated to us, from Shaqiq, from Umm Salamah, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "When you are present with a sick person or a dying person, say good things, for the angels say 'Amin' to what you say." She said: When Abu Salamah died, I said: "O Messenger of Allah, what should I say?" He said: "Say: 'O Allah, forgive us and him, and grant me a good successor from him.'" And Ibn Numayr said: "A righteous one." She said: So Allah, the Mighty and Majestic, granted me Muhammad, peace and blessings of Allah be upon him, as a successor from him.