Is it essential that the du’aa’s recited in prayer be proven in the Sunnah?
Question

                When praying qiyaam al-layl, if the worshipper bows and prostrates for a long time, does he have to recite all the adhkaar that are narrated from the Prophet (peace and blessings of Allaah be upon him) or can he say whatever du’aa’ he likes?.
Praise be to Allah.
    There is nothing wrong with a worshipper saying whatever 
    du’aa’ he wants in his prayer, those that are narrated in the Sunnah – which 
    are the best and most comprehensive of du’aa’s – or any kind of permissible 
    du’aa’ that he wishes. 

    The one who recites du’aas that are narrated in sharee’ah 
    will be rewarded for following what is narrated in sharee’ah, and he will be 
    rewarded for his du’aa’. 

    Among the texts which show that du’aa’ in general is 
    permissible is the words of the Prophet  (peace and blessings of Allaah 
    be upon him): “Then let him choose whatever supplications he wishes.” 
    According to another report: “Then let him choose whatever du’aa’ he likes 
    and say it.” Narrated by al-Bukhaari (835) and Muslim (402). This was 
    narrated concerning du’aa’ before the salaam at the end of the prayer. 

    Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) 
    said: 

    It is prescribed for the believer to say du’aa’ during his 
    prayer in the places when du’aa’ is allowed, whether the prayer is 
    obligatory or supererogatory (naafil). The places where du’aa’ is allowed 
    are prostration, between the two prostrations and at the end of the prayer 
    after the tashahhud and blessings upon the Prophet  (peace and blessings 
    of Allaah be upon him) and before the tasleem, as it is proven that the 
    Prophet  (peace and blessings of Allaah be upon him) used to pray for 
    forgiveness between the two prostrations and it is proven that he used to 
    say between the two prostrations: “Allaahumma ighfir li wa’rhamni 
    wa’hdini wa’jburni wa’rzuqni wa ‘aafini (O Allaah, forgive me, have 
    mercy on me, guide me, enrich me, grant me provision and keep me safe and 
    sound).  

    And he  (peace and blessings of Allaah be upon him) said: 
    “As for bowing, glorify the Lord therein, and as for prostrating, strive 
    hard in du’aa’ therein because it is more likely that you will receive a 
    response.” Narrated by Muslim in his Saheeh. Muslim also narrated 
    from Abu Hurayrah (may Allaah be pleased with him) that the Prophet  
    (peace and blessings of Allaah be upon him) said: “The closest that a person 
    is to his Lord is when he is prostrating, so say a lot of du’aa’ then.” 

    In al-Saheehayn it is narrated from ‘Abd-Allaah ibn 
    Mas’ood (may Allaah be pleased with him) that when the Prophet  (peace 
    and blessings of Allaah be upon him) taught him the tashahhud he said: “Then 
    let him choose whatever supplications he wishes.” According to another 
    report: “Then let him choose whatever du’aa’ he likes and say it.” 

    There are many ahaadeeth that speak of this, and they 
    indicate that it is prescribed to say du’aa’ in these places in the prayer, 
    whether it has to do with the Hereafter or with worldly matters, so long as 
    the du’aa’ does not involve any sin or severing of family ties. And it is 
    better to say a lot of du’aa’s that were narrated from the Prophet  
    (peace and blessings of Allaah be upon him). End quote. 

    Fataawa al-Shaykh Ibn Baaz 
    (11/171, 172). 

    He (may Allaah have mercy on him) also said: 

    There is nothing wrong with saying du’aa’ during prayer, 
    whether that is for oneself or for one’s parents or anyone else. Rather that 
    is prescribed, because the Prophet  (peace and blessings of Allaah be 
    upon him) said: “The closest that a person is to his Lord is when he is 
    prostrating, so say a lot of du’aa’ then.” … Then he quoted the ahaadeeth 
    that we have mentioned above, then he said: 

    So if he says du’aa’ when he is prostrating or at the end of 
    the prayer, for himself or for his parents or for the Muslims, there is 
    nothing wrong with that, because of the general meaning of these ahaadeeth 
    and others. End quote. 

    Fataawa al-Shaykh Ibn Baaz 
    (11/173, 174). 

    And Allaah knows best.