Musnad Abu Ya'la al-Mawsili 209:86

Abu Khaythama narrated to us, Jarir narrated to us, from al-A'mash, from Shaqiq, from Umm Salama, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When you are present with a sick person or a dying person, say good things, for the angels say 'Amin' to what you say." When Abu Salama died, I came to the Prophet (peace and blessings of Allah be upon him) and said: "What should I say?" He said: "Say: 'O Allah, forgive us and him, and grant us a good successor from him.'" So I said it, and Allah (Mighty and Majestic is He) granted me Muhammad (peace and blessings of Allah be upon him) as a successor.