The Prose Edda 2.16

"How should one periphrase Loki? Thus: call him Son of Fárbauti and Laufey, or of Nál, Brother of Býleistr and of Helblindi, Father of the Monster of Ván (that is, Fenris-Wolf), and of the Vast Monster (that is, the Midgard Serpent), and of Hel, and Nari, and Áli; Kinsman and Uncle, Evil Companion and Bench-Mate of Odin and the Æsir, Visitor and Chest-Trapping of Geirrödr, Thief of the Giants, of the Goat, of Brísinga-men, and of Idunn's Apples, Kinsman of Sleipnir, Husband of Sigyn, Foe of the Gods, Harmer of Sif's Hair, Forger of Evil, the Sly God, Slanderer and Cheat of the Gods, Contriver of Baldr's Death, the Bound God, Wrangling Foe of Heimdallr and of Skadi. Even as Úlfr Uggason sings here:

The famed rain-bow's defender, Ready in wisdom, striveth At Singasteinn with Loki, Fárbauti's sin-sly offspring; The son of mothers eight and one, Mighty in wrath, possesses The Stone ere Loki cometh: I make known songs of praise.

Here it is written that Heimdallr is the son of nine mothers.