When should the man I intend to follow in prayer be informed: before the intention of prayer or after?

It is permissible for someone who wishes to pray behind another person to inform him before or after the prayer. It is preferable to inform him beforehand so that the Imam may receive the reward for the congregational prayer. If the Imam does not intend to lead the prayer (imamate), the prayer of the follower is still valid, and he receives the reward for the congregational prayer. It is not a condition for the validity of following an Imam that the Imam intends to lead the prayer; the correct and well-known opinion, which the majority have affirmed, is that it is not obligatory. As for whether the Imam's prayer becomes a congregational prayer if he did not intend it to be so, some say he does not attain the virtue of congregational prayer, while others say he does attain it if a group follows him and he is unaware of them, because they attained it through him.