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Ali al-Sajjad, also known as Ali ibn Husayn, was the fourth Shia Imam. He is revered for his piety and is known for his eloquent prayers, which were collected into a book called the "Sahifa al-Sajjadiyya." Ali al-Sajjad's nickname, "Sajjad," means "the one who prostrates frequently," reflecting his deep devotion to prayer.