The Habit of Reciting Certain Surahs of the Qur'an in the Tahajjud Prayer

Q: What is the ruling on the habit of reciting (Subha man rabbika al-a'la), (Qul ya ayyuha al-kafirun), and (Qul huwa Allahu ahad) in the last three rak'ahs of the tahajjud prayer? And what is reported regarding the recitation of the last three surahs of the Qur'an in the final rak'ah with which one performs witr?

A: This is the best, but if one occasionally leaves it off so that people know it is not obligatory, that is good. Otherwise, the best is to follow the example of the Prophet (peace and blessings be upon him), for he used to recite (Subha), (al-Kafirun), and in the three rak'ahs with which he performed witr.
But if a person occasionally leaves it off so that people know it is not required, just as some of the Salaf said regarding leaving the recitation of Surat al-Sajdah and occasionally in the Fajr prayer on Friday, in order to inform people that it is not required, otherwise the sunnah is to recite them in the Fajr prayer every Friday. But if the imam occasionally leaves it off so that people know this is not obligatory, then there is nothing wrong with that.
For example: leaving the recitation of (Subha), (al-Kafirun), and in the three rak'ahs with which one performs witr, as mentioned earlier, so that people know that reciting them is not obligatory. But the best is to recite them frequently, and for that to be the predominant practice.
As for what is reported regarding the recitation of the last three surahs of the Qur'an, it is weak. The preserved report is that one recites only Surat after al-Fatihah in the rak'ah with which one performs witr.