Abū Bakr Aḥmad ibn Ḥusayn ibn ʿAlī ibn Mūsā al-Khusrawjirdī al-Bayhaqī (Persian: ابو بكر احمد بن حسين بن علي بن موسی خسروجردی بيهقی, 994–1066), also known as Imām al-Bayhaqī, was a Sunni scholar widely known for being the foremost leading hadith master in his age, leading authority in the Shafi'i school, leading authority on the foundation of doctrine, meticulous, a devoted ascetic and one of the notable defenders of the Ash'ari school. Al-Dhahabi said: "Unequalled in his age, unrivalled amongst his peers, and the Ḥāfiẓ of his time."

Al-Bayhaqi was born c. 994 CE/384 AH in the small town of Khosrowjerd near Sabzevar, then known as Bayhaq, in Khorasan.

Al-Bayhaqi spent his early years and childhood in the city of Bayhaq before moving elsewhere to pursue his studies. He journeyed throughout Khorazan, Iran, Iraq, Hejaz, and other Muslim countries out of a passion for knowledge. He narrated Hadith from numerous scholars during his intensive travels.

He studied fiqh under two prominent jurists, Abū al-Fatḥ Nāṣir ibn al-Ḥusayn ibn Muḥammad al-Naysaburi as well as Abul Hasan Hankari. He studied hadith under Hakim al-Nishaburi (foremost leading hadith scholar at his time) and was al-Nishaburi's foremost pupil as well as extensively studying hadith under Abu Ishaq al-Isfarayini, Abu Bakr al-Barqani, and many others. Al-Bayhaqi belongs to the third generation of Ash'ari school and took kalam from two prominent theologians, Ibn Furak and Abu Mansur al-Baghdadi. He took his Tasawwuf as well as narrating hadith from Al-Sulami. He had the privilege to study kalam under Al-Halimi in his younger days.

Al-Bayhaqi taught and authored books in Bayhaq and lived most of his life there, until 1049, when he was asked to go to Nishapur. This request resulted from Al-Kunduri, the vizier of Tughril Beg, the Seljuq sultan who had been ruling from Nishapur since 1040, persecuting the Ash'arites. Al-Kunduri's motive for this attack was probably his competition with Aby Sahl Ibn al-Muwaffaq for the position of vizier; the latter was a Shafi'ite by law whereas al-Kunduri was a Hanafite. Two distinct Ash'arites, al-Qushayri and al-Juwayni, were barred from the mosques in or around 1049. The Sultan then issued an order imprisoning them, as well as Abu-Sahl Ibn al-Muwaffaq and other individuals. The two theologians eventually travelled to the Hijaz and left Nishapur. In the meantime, al-Bayhaqi had written a lengthy letter to al-Kunduri that has survived, demonstrating the Ash'arites' immunity from heretical accusations and requesting an end to the persecution. Al-Bayhaqi travelled to Nishapur approximately a year later; he may have stayed there for a few years prior to travelling to the Hijaz. Around the years 1063 and 1064, following al-Kunduri's imprisonment and eventual death, the exiles returned to Nishapur.

Al-Bayhaqi had large number of students. He would give them the authority to narrate and teach his books. His most famous pupils associated with him include:

He died in Nishapur on the 10th of Jumādā al-ťlā, 458 AH corresponding to the 9th April 1066 at the age of 74. He was washed, put in a coffin, and transported [two days at the time] to Bayhaq, where he was buried.

Al-Bayhaqi was known for his extreme piety and was a frugal spender in the same manner of the pious scholars. He constantly fasted for thirty years straight before his death except the days of Eid and Tashriq which are prohibited to fast. It is known that perpetual fast (Sawm al-Dahr) is a famous practise done by several companions and the Salaf such as Umar, Uthman, Ibn Shihab al-Zuhri, Abu Hanifa, Al-Shafi'i, Al-Tustari, etc.