Me and a friend were praying together and it came to the last unit of prayer and I had waswas of whether I did one or two prostrations so I thought it better to do the prostrations of forgetfulness but I didn't know whether my friend had to do them with me and I thought to do them alone and then say salam and I did and he did not follow me.. I later checked online and it said he had to do them with me.. and so I told him to repeat the prayer and he didn't agree to repeat it. What do I do in this case? and what should he do in this case?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.

We would like to first direct our response to you before answering the question about your friend. You should have gone back and performed the prostration which you had doubted performing during the prayer before performing Sujoold As-Sahw (the two prostrations of forgetfulness), because if someone doubts whether he performed any of the prayer's pillars, it is obligatory for him to go back and perform it, unless it seemed most likely that he had performed it. In the latter case, the most evident opinion is that he is not obliged to perform it. Al-Hajjaawi wrote: "If the worshipper doubts whether he left out a pillar (Rukn) of the prayer, then it is the same as actually leaving it out ..." [Zaad Al-Mustaqni']

Ibn Al-‘Uthaymeen underlined the same point in his book Ash-Sharh Al-Mumti‘: "The statement, 'If the worshipper doubts leaving out a pillar of the prayer, then it is the same as actually leaving it out,' means that if someone doubts whether he performed a pillar of the prayer, then the same ruling on leaving out a pillar of the prayer applies, i.e. it is incumbent on him to perform it and perform two prostrations of forgetfulness. This is because uncertainty of performing it is just like leaving it out in this case. However, if it seemed most likely to him that he had performed it, then the most evident opinion is that he is not obliged to do anything because it is as if he had performed it."

Therefore, if you did not perform the prostration about which you had doubts and settled merely for performing the two prostrations of forgetfulness, then it is incumbent on you to repeat that prayer because you had failed to perform a pillar of the prayer.

Also, you should have performed the two prostrations of forgetfulness after Tasleem not before it. Az-Zarkashi wrote: "Where it seems most likely to the worshipper that he had performed the pillar of the prayer (i.e. in case of doubtful certainty), he is obliged to perform the two prostrations of forgetfulness after Tasleem ..." [Sharh Mukhtasar Al-Khiraqi] '

This is the opinion adopted by the majority of scholars other than the Shaafi‘is.

As for your friend, in principle, those who are led in prayer are obliged to follow the Imaam in performing the prostrations of forgetfulness and if they do not follow the Imaam in this regard, their prayer is invalid unless they know the reason the Imaam is prostrating and know with certainty that he is mistaken. In this case, they should not follow him. Imaam An-Nawawi wrote: "If the Imaam made a mistake that requires prostrations of forgetfulness, those being led in prayer should follow him in performing the prostrations of forgetfulness except in two cases: First, if it is clear that the Imaam's Wudhoo' (ablution) has been invalidated (during the prayer), those being led are not obliged to follow the Imaam in performing the prostration of forgetfulness, and if they failed to do any act of the prayer, the Imaam's prayer is not sufficient for them. Second, if those being led know the reason the Imaam is performing the prostrations of forgetfulness, and they know with certainty that he is mistaken in his doubt (that he did not leave anything off) or if the Imaam recited something out loud while it should not have been recited out loud or vice versa, then those being led should not follow the Imaam in performing the prostration of forgetfulness. However, it is incumbent on those being led to follow the Imaam in everything besides these two exceptional cases. If those being led deliberately failed to follow the Imaam, their prayer is invalid. Whether they realized the Imaam's forgetfulness or not, whenever the Imaam performed the prostrations of forgetfulness at the end of the prayer, they have to follow him assuming that he was forgetful." [Al-Majmoo‘]

Therefore, you should examine your prayer and that of your friend in the light of the above-mentioned explanation. If you know that your prayers were valid, there is no problem; otherwise, you are obliged to repeat them.

Allaah Knows best.