What is the ruling for patting someone’s shoulder signalling that we want to join him in congregational prayer (thus, making him our imam)?
Alhamdulillah, praise and thanks to Allah for the many countless blessings He has blessed us all with. Blessings and salutations to the Prophet Muhammad PBUH, his wives, his family, companions and all those that follow his teachings to the day of judgement.
In our opinion, it is not necessary to pat someone’s shoulder as a sign that we are joining him in congregational prayer. It is enough to have the intention and just follow him in prayer. In this situation, even if someone is performing a sunnah prayer, then if someone pats his shoulder, he should continue on with his prayer and it is not necessary for him to change his intention to be an imam. See Mughni al-Muhtaj by al-Syarbini al-Khatib (1/503). Hence, it is permissible to follow someone in prayer without patting his shoulder.