Question 20: If a person joins Salaah (Prayer) during the last Tashahhud (testification recited in the sitting position in the last unit of Prayer) before the Imaam (the one who leads congregational Prayer) pronounces Tasleem (salutation of peace ending the Prayer), will they get the reward of offering congregational Salaah? Or will they get the reward of praying individually? What is the best course of action upon entering the Masjid (mosque) and finding the Imaam in the last Tashahhud? Should a person complete the Tashahhud, or should they wait for others to come and pray with?
Answer: Anyone who joins the Imaam during the last Tashahhud will not be considered to have caught the congregational Salaah, but they will get the reward of whatever parts of Salaah they pray with the Imaam. To be considered having joined the congregational Salaah, a latecomer should offer at least one Rak`ah (unit of Prayer) with the Imaam, for the Prophet (peace be upon him) said... read more here.