What is the ruling on one who sits in the Tarawih prayer until the imam is about to bow, at which point he stands, says takbir, and bows?

Question:
Honorable Shaykh, may Allah grant you success. He says: What is the ruling on one who sits in the Tarawih prayer until the imam is about to bow, at which point he stands, says takbir, and bows, having not recited al-Fatihah? Is the prayer of such a person valid?

Answer:
This is not valid because he left the pillar of standing. He left the pillar of standing while he was able to perform it. Standing is only dropped for one who did not catch up with the imam; if he came late and only caught the bowing, he bows with him and standing is dropped from him because he missed it. But as for a person who sits and intends this, and does not stand except at the bowing, this rak'ah does not suffice for him because he left a pillar without an excuse. Since the prayer is a voluntary prayer, such as Tarawih or Tahajjud, he is able to say takbir and recite al-Fatihah while sitting, because it is permissible for one performing a voluntary prayer to sit. But for the obligatory prayer, no, standing is required. At the very least, they say to the extent of reciting al-Fatihah. Standing before the bowing is required if he is able to do so, to the extent of reciting al-Fatihah.