The Imam

Imam is the one who is the most versed in the Quran:

Ibn Mas'ud reports that the Prophet sallallahu alehi wasallam said:

The Imam of a people should be the one who is the most versed in the Book of Allah.
If they are equal in their recital, then the one who is most knowledgeable of the sunnah.
If they are equal in the sunnah, then the one who migrated first.
If they are equal in that, then the eldest.

Abu Hurairah reports that the Prophet sallallahu alehi wasallam said: " It is not allowed for a man who believes in Allah and the last day to be an Imam for a people except with their permission. This is related by Abu Dawud

When the people dislike their Imam

Ibn 'Abbas relates that the Prophet s.a.w. said: " Three people's prayers will not rise above their head the length of a head's span:

a man who leads a people in salah , and they do not like him
a woman who has disobeyed her husband , and he is displeased with her
and two brothers who are estranged." This is related by Ibn Majah.

'Abdullah Ibn 'Amr relates that the Prophet sallahu alehi wasalam said: " Allah s.w.t does not accept prayers from three people:

a man who leads a people and they dislike him,
a man who attends the prayers after their time is finished ,
and a man who re-enslaves his freed slaves." This is related by Abu Dawud & Ibn Majah.

Elaborating upon it, at Tirmidhi says:" It is dislike that a man should lead a people in salah while they dislike him. If the Imam is not a wrongdoer, then the sin is upon those who don't like him.