What must we do after the Imam prostrated once for forgetfulness instead of twice, then we dispersed before he could make up the second prostration?

Whoever forgets the first tashahhud in their prayer, if they remember it after having passed the rukn (pillar) of standing, they should not return to it but instead perform the prostration of forgetfulness (sujud as-sahw). However, if they remember it before fully standing upright, they should sit for the tashahhud and then perform the prostration of forgetfulness. The prostration of forgetfulness consists of two prostrations, not one.

If the Imam performs only one prostration of forgetfulness out of ignorance, his prayer and the prayer of those behind him are valid. The congregants should perform the second prostration if their Imam did not.