Leading the prayers by one who does not do rukoo’ properly

The one who is most entitled to lead the prayers is the one
who has most knowledge of the Book of Allaah and who has most knowledge of
the fiqh of prayer and is most keen to ensure that his prayer is correct.
That includes doing the essential parts and obligations of the prayer in the
required manner. But if he does not do the prayer or one of its essential
parts, such as rukoo’, properly, then the matter is subject to further
discussion: 

-If he is unable to stand or bow
or prostrate for a valid reason such as sickness etc., then there is a
difference of opinion among the scholars. The most correct view is that his
leading the prayers is valid because of the general meaning of the hadeeth
of the Prophet (peace and blessings of Allaah be upon him): “The people
should be led in prayer by the one who has most knowledge of the Book of
Allaah.” This is the madhhab of al-Shaafa’i (may Allaah have mercy on him),
as it says in al-Majmoo’, 4/264, and this was the view favoured by
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) as was narrated
from him in al-Insaaf, 4/374.

If the reason is ignorance or stubbornness,
then his mistake should be pointed out to him. If he responds, all well and
good, otherwise it is not permissible for him to lead the people in prayer,
even if he is hafiz, because his prayer is invalid due to his failing to do
one of the essential parts of prayer. See Question no.
1875.