Cracking the knuckles is makrooh whilst praying
Question

                What is the interpretation of the hadith that talked about cracking the knuckles? Is forbidden both during and outside prayers?
Praise be to Allah.
    Cracking the knuckles is makrooh whilst 
    praying, not all the time. Rather it is makrooh during prayer, because it 
    will either distract the one who does it from the prayer, or it will disturb 
    other worshippers. 

    The hadeeth that was narrated concerning 
    that is not the words of the Prophet (blessings and peace of Allah be upon 
    him); rather it is mawqoof and its isnaad ends with ‘Abdullah ibn ‘Abbaas 
    (may Allah be pleased with him) and is his words. 

    It was narrated that Shu‘bah, the freed 
    slave of Ibn ‘Abbaas, said: I prayed beside Ibn ‘Abbaas and cracked my 
    knuckles, and when I had finished praying, he said: May you have no mother! 
    [This is a word of rebuke] You crack your knuckles whilst you are praying?

    Narrated by Ibn Abi Shaybah (2/344). Al-Albaani 
    said in Irwa’ al-Ghaleel (2/99): Its isnaad is hasan. 

    Shaykh Ibn ‘Uthaymeen (may Allah have 
    mercy on him) was asked: Does cracking the knuckles absentmindedly whilst 
    praying invalidate the prayer? He replied: Cracking the knuckles does not 
    invalidate the prayer, but cracking the knuckles is a kind of fidgeting, and 
    if that is during a congregational prayer, it will inevitably disturb those 
    who hear it, and that is more bothersome than if there is no one around him.

    End quote from Fataawa Arkaan al-Islam 
    (p. 341) 

    The Standing Committee for Issuing Fatwas 
    was asked (fatwa no. 21349): The Prophet (blessings and peace of Allah be 
    upon him) forbade interlacing the fingers in the mosque; does that include 
    cracking the knuckles? Please note that there is no hadeeth which forbids 
    it. 

    They replied: A number of the scholars 
    stated that cracking the knuckles is makrooh inside the mosque, and they 
    connected it to interlacing the fingers, because both come under the heading 
    of fidgeting.

    End quote from Fataawa al-Lajnah ad-Daa’imah 
    – 2 (5/266) 

    See also the answer to question no. 
    36801

    And Allah knows best.