Which caliph's Cordoba library allegedly held 400,000 volumes?

Al-Hakam II, also known as Abū al-ʿĀṣ al-Mustanṣir bi-Llāh al-Hakam b. ʿAbd al-Raḥmān, was the Caliph of Córdoba. He was the second Umayyad Caliph of Córdoba in Al-Andalus, and son of Abd-al-Rahman III and Murjan. He ruled from 961 to 976.