Both leading the ṣalāh and being led [in ṣalāh] are actions that require intention. This is because congregation is considered an additional circumstance to the fundamental action of ṣalāh. For this reason, we say: The intention of praying in congregation is obligatory prior to starting the performance of congregational prayer, it is not simply achieved by praying it. Thus, does this [additional] circumstance [of leading others or being led] require a specific intention, or would it be sufficient for a person to simply follow the actions of another? The answer to this is provided by our statement: “Both leading the ṣalāh and being led [in ṣalah] are actions that require intention”. That is, the imām must intend by his ṣalāh to lead those behind him, just as those being led must intend to be led in ṣalāh. The evidence of this is the saying of the Prophet (صلى الله عليه وسلم): “Indeed, actions are judged by their intentions and every man shall have only what he intended”. There should, therefore, be no doubt that intention is a prerequisite for the attainment of the reward of praying in congregation for both the imām and those who pray behind him. The reward of praying in congregation is not achieved unless the imām intends to lead those behind him and those behind him intend to be led.

However, is this intention considered a prerequisite upon which the validity of the ṣalāh itself is predicated? The aforementioned statement explicitly indicates that the intention for congregation is indeed a prerequisite for the validity of the ṣalāh itself. Therefore, if the imām does not intend by his actions to lead others, or those who follow his ṣalāh behind him do not intend to be led by him, then both the ṣalāh of the imām and those behind him should be considered invalid. This may be clarified in consideration of the following scenarios:

1. An imām intends to be led by another, and the one being led intends by his actions to lead others. This is incorrect, due to the inherent contradiction in roles and because the actions of the imām are different to the actions of those being led [like saying سمع الله لمن حمده which is only said by the imām in congregational prayers].
2. Both parties intend by their actions to lead the other. This is also incorrect due to evident contradiction. One cannot be led by another while simultaneously being the imām.
3. Both parties intend to be led by the other. Again, this would be incorrect due to contradiction in action. Also, if both parties are being led by the other, who is the imām?
4. The ones being led intend to follow the actions of a person they have designated as imām, but this person has no intention to lead them. Here, the ṣalāh of those intending to be led is incorrect, while the ṣalāh of the one they designated would be deemed correct. An example of this would be if a praying person is approached by another who begins to follow his actions as if this person is his imām. This person who was already praying never intended to be the imām for the other. Here, the first person’s ṣalāh would be deemed correct, and the ṣalāh of the person who has chosen to follow him is incorrect. This is because he intended to be led by one who did not intend to act as his imām. This is the opinion of the Ḥanbalī madh`hab and among its singular positions, as expressed in ‘al-Inṣāf’.Another opinion in this issue is that it is considered correct for a person to designate an imām without him intending to lead. The proponents of this opinion support their stance with the ḥadīth in which the Prophet (صلى الله عليه وسلم) stood to pray one night during Ramaḍān causing the people to gather and pray with him despite the fact that he (صلى الله عليه وسلم) was unaware of their presence. Then, when he prayed on the second and third nights he was aware of them. On the fourth night, he (صلى الله عليه وسلم) did not lead them out of fear that this prayer may be made obligatory upon them. This is the stance of Imām Mālik (رحمه الله) and it is more correct.Also, because the wisdom behind designating an imām is following his actions which have occurred [despite him not intending to lead]. In this circumstance, those being led would have attained the reward of praying in congregation, while the imām would not have attained this reward. This is because those being led intended to follow an imām, so they are deserving of the reward of their intention. As for the designated imām, he never intended to lead others and is therefore undeserving of the reward associated with leading.
5. A person intends by his actions to lead others without being followed by those around him. For example, a man stands next to another and says the takbīr loudly while surmising that this person wants to be led by him. Here, this man intends to lead another, but the one(s) he intends to lead has not taken him as imām. In this situation, the reward of praying in congregation will remain unachieved by both the imām and the one he intends to lead. As, in reality, there is no congregation. The one being led is not following this imām, nor has he designated him as such. As for the imām, he has intended to lead while not being followed by anyone. There can be no reward for praying in congregation without the presence of a congregation. Although if a person was to adopt the opinion that the imām here is deserving of reward, it would not be far from the truth due to the encompassing nature of his (صلى الله عليه وسلم) saying: “Indeed, actions are judged by their intentions and every man shall have only what they intended”.
6. A person follows another without the intention of being led. In this circumstance, the reward of praying in congregation is unattained as he has not intended to pray in congregation. This occurs if a person prays behind an imām whose ṣalāh is incorrect. He follows him out of shyness without intending to be led. Or, for example, if a person praying behind another nullifies his wuḍūʾ but is embarrassed to leave the congregation and perform wuḍūʾ, so he continues to follow along with the actions of the congregation, while not intending to pray due to his lack of purity. This occurs despite being impermissible. As, in such a situation, a person is obligated to abandon the ṣalāh immediately, perform wuḍūʾ, and restart his prayer from the beginning.

Source: Al-Sharḥ al-Mumtiʿ 2:304-307
Translated by: Riyāḍ al-Kanadī