Is my prayer valid and sufficient, or must I repeat it? And what is the ruling on delaying behind the Imam in prayer, given that I performed three prostrations in the last rak’ah, then performed the prostrations of forgetfulness (sujud al-sahw), then recited the tashahhud of forgetfulness before the taslim, and did not complete the tashahhud?

The prayer of one who performed three prostrations by mistake in the last rak'ah, then performed the prostration of forgetfulness (sujud al-sahw), then recited the tashahhud by mistake after sujud al-sahw before saying the taslim, and then remembered and cut short the tashahhud and said the taslim, is valid. What is prescribed for him is sujud al-sahw for the extra prostration, and performing it before the taslim is a correct action. Reciting the tashahhud after sujud al-sahw is a matter of scholarly disagreement, and performing part of the tashahhud and then cutting it short before completing it does not invalidate the prayer. Following (the imam) is the Sunnah for the ma'mum (follower), and its meaning is to begin the actions of prayer immediately after the imam begins them, without coinciding with him. What was mentioned about the imam rising from ruku' and the ma'mum following him is not considered a delay but rather following.