Making the second rak’ah long and the first rak’ah short
Question

                What is the ruling on praying behind an imam who makes the first rak’ah short and makes the second rak’ah long?.
Praise be to Allah.The Sunnah in prayer is 
    to make the first rak’ah long and the second short, because of the report 
    narrated by al-Bukhaari (759) and Muslim (685) from Abu Qataadah (may Allaah 
    be pleased with him) who said: The Prophet  (peace and blessings of 
    Allaah be upon him) used to recite in the first two rak'ahs of Zuhr prayer 
    the Opening of the Book (al-Faatihah) and two soorahs, making the first 
    rak’ah long and the second short, and he made his recitation audible 
    sometimes. In ‘Asr he used to recite the Opening of the Book and two 
    soorahs, making the first rak’ah long. And he used to make the first rak’ah 
    of Fajr prayer long and the second short. 

    Al-Bukhaari (may Allaah have mercy on him) included this 
    hadeeth in a chapter in his Saheeh entitled, “Chapter on making the 
    first rak’ah long.” In this chapter he reported the hadeeth of Abu Qataadah 
    which says that the Prophet  (peace and blessings of Allaah be upon him) 
    used to make the first rak’ah long in Zuhr prayer and the second rak’ah 
    short, and he also did that in Fajr prayer. 

    Al-Haafiz ibn Hajar said: The phrase  “Chapter on making the 
    first rak’ah long” refers to all the prayers, which is apparent from the 
    hadeeth mentioned. End quote from Fath al-Baari, 2/305 

    Al-Nawawi (may Allaah have mercy on him) said: The phrase “he 
    used to make the first rak’ah long in Zuhr prayer and the second rak’ah 
    short” is something concerning which the scholars differed with regard to 
    following the apparent meaning. There are two views among our companions, 
    the more well known of which is that it should not be made lengthy, and the 
    hadeeth is to be understood as meaning that it was made long because of the 
    opening du’aa’ and prayers for refuge with Allaah, or because he sensed that 
    someone was about to join the prayer, and so on, not because of recitation. 
    The second view is that it is mustahabb to make the recitation long in the 
    first rak’ah on purpose. This is the correct view which is in accordance 
    with the Sunnah. End quote from Sharh Muslim, 4/175 

    Based on this, if the imam makes the second rak’ah longer 
    than the first, and he always does that and makes it a habit, then this is 
    contrary to the Sunnah, and he should be advised to follow the Sunnah of the 
    Prophet  (peace and blessings of Allaah be upon him), for the best 
    guidance is the guidance of Muhammad  (peace and blessings of Allaah be 
    upon him). But making the first rak’ah long than the second is not 
    obligatory and is not an essential condition of the prayer being valid, 
    rather it is preferable.