Cearl is the first Mercian king mentioned by which English monk at the monastery of St. Peter and its companion monastery of St. Paul in the Kingdom of Northumbria of the Angles?
Cearl of Mercia:  He is the first Mercian king mentioned by Bede in his "Historia ecclesiastica gentis Anglorum". Bede: Bede ( ; Old English: "Bǣda" or "Bēda" ; 672/3 – 26 May 735), also known as Saint Bede, Venerable Bede, and Bede the Venerable (Latin: "Bēda Venerābilis" ), was an English monk at the monastery of St. Peter and its companion monastery of St. Paul in the Kingdom of Northumbria of the Angles (contemporarily Monkwearmouth–Jarrow Abbey in Tyne and Wear, England).
Bede