They did not establish the Prophetic Sunnah, nor did they practice the 'Umaric Sunnah in "tarawih."
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The Shaykh : Those who hold that it is permissible to increase, among them Shaykh al-Islam Ibn Taymiyyah (may Allah have mercy on him), says to you: when the people's resolve weakened, they substituted one thing for another. They could no longer stand for long, so they divided this prayer into two parts. For example, the imam used to recite a part in two rak'ahs, or he would recite in four rak'ahs. We observe in this reasoning that if it were a Shari'ah text, it would mean they contradicted this text!
Abu Malik: Well, of course... backward.
The Shaykh : So they increased the number of rak'ahs as a substitute for what? The lightening of the recitation. They lightened it in number and in quality. So they did not establish the Prophetic Sunnah, and they did not practice the 'Umaric Sunnah, if it is authentic. Therefore, these people are following only customs and desires.
Abu Malik: Well, of course... backward.
The Shaykh : So they increased the number of rak'ahs as a substitute for what? The lightening of the recitation. They lightened it in number and in quality. So they did not establish the Prophetic Sunnah, and they did not practice the 'Umaric Sunnah, if it is authentic. Therefore, these people are following only customs and desires.