Muhaqqaq Masala

A Masbooq completing the remaining prayer at someone else's instance

If a Masbooq turns the Salam along with the Imam, and then completes his prayer based on someone else's telling, his prayer will become invalid (Fasid). However, if he remembered after turning the Salam (even if this remembering occurred just by looking at the person praying next to him, and then he stood up), the prayer will not become invalid. (Aham Masail / Vol: 2 / Pg: 85)