Mukhtasar Muslim (Abridged) 284 — Reciting “Alhamdulillah” and saying

Narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: When the Imam says Ameen, the followers should also say Ameen, and whoever's Ameen coincides with the Ameen of the angels, his past sins will be forgiven. Ibn Shihab (may Allah have mercy on him) stated that the Messenger of Allah (peace and blessings of Allah be upon him) used to say Ameen after (the recitation of) "wa la-d-dallin."