Al-Sahifat al-Sajjadiyya is the oldest prayer manual in Islamic sources and one of the most seminal works of Islamic spirituality of the early period. It was composed by the Prophet's great grandson, 'Ali ibn Husayn, known as Zayn al-'Abidin ("the adornment of the worshippers"), and has been cherished in Shi'ite sources from earliest times. Zayn al-'Abidin was the fourth of the Shi'ite Imams, after his father Husayn, his uncle Hasan, and his grandfather 'Ali, the Prophet's son-in-law. Shi'ite tradition considers the Sahifa a book worthy of the upmost veneration, ranking it behind only the Qur'an and 'Ali's Nahj al-Balagha.