From Aleteia:
St. Anne is portrayed with her finger to her lips in this 8th- or 9th-century fresco, discovered in old Nubia in present-day Sudan.A unique portrait of St. Anne, mother of the Blessed Virgin Mary, was discovered in the 1960s by a Polish archaeological team during an excavation of the Faras Cathedral in old Nubia, in present day Sudan.
According to Archaeotravel.eu, the gesture, which is rarely seen in Christian art, may refer to a Egyptian Christian (Coptic) tradition in which people prayed quietly while holding a finger of the right hand on one’s lips. The gesture was supposed to protect people from the evil that would try to enter the human heart during prayer.