What is the ruling on my prayer if I missed part of the 'Isha prayer, then joined a congregation that had already prayed three Rak'ahs, and I did not recite Al-Fatihah in the fourth Rak'ah with the Imam because I thought time was short, then it became clear that the Imam repeated the entire prayer? Am I sinful for not alerting the Imam to his ignorance of the ruling? And will I be held accountable if I see someone who does not pray properly and I do not alert them?

We previously discussed the ruling on teaching the ignorant. As for the congregant (ma'mum), it is obligatory for him to alert his Imam to his error. However, alerting someone other than the Imam is a matter of scholarly disagreement. Regarding your pursuit of the other congregant and your attempt to call out to him, you erred in that. What was obligatory upon you was to rectify your prayer, because your preoccupation with pursuing the man and calling out to him prolonged the separation between the remaining rak'ah and the rest of the prayer. In addition, there was movement that was not part of the prayer and uttering extraneous speech, which makes building upon your previous prayer impossible. You would be in the position of one who cut off the prayer, and it is not permissible to cut off the prayer without a legitimate religious justification. Your prayer was nullified by your extensive, successive movements that were not part of the prayer, and you are obligated to restart the prayer. If you did not do so and only intended to complete your prayer, you are obligated to repeat it. If you repeated it, then your responsibility has been discharged. We have previously clarified the ruling on separating from the Imam when he hurries in prayer.