What is the ruling on one who pronounced the Tasleem to end his prayer before his Imam, after hearing the Tasleem of a worshipper who entered the mosque? And is the one who caused that held sinful?

If one says the taslim (salutation of peace) before their Imam out of forgetfulness, they should wait for their Imam's taslim and say it with him. No prostration of forgetfulness (sujood as-sahw) is required of them, and their prayer is valid. However, if they intentionally say the taslim before their Imam, their prayer is nullified.