Kathasaritsagara 3.6.84

Then the goddess distracted with anger and grief, said, " I have not obtained a son from you after all," and Śiva said to her; "An obstacle has arisen in this matter, because you neglected to worship Ganésa, the lord of obstacles; therefore adore him now in order that a child may be born to us of the fire." "When thus addressed by Śiva, the goddess worshipped Ganésa, and the fire became pregnant with that germ of Śiva. Then, bearing that embryo of Śiva, the fire shone even in the day as if the sun had entered into it. And then it discharged into the Ganges the germ difficult to bear, and the Ganges, by the order of Śiva, placed it in a sacrificial cavity on mount Meru. There that germ was watched by the Ganas, Śiva's attend and after a thousand years had developed it, it became a boy with six faces. Then, drinking milk with his six mouths from the breasts of the six Krittikás appointed by Gaurí to nurse him, the boy grew big in a few days. In the meanwhile, the king of the gods, overcome by the Asura Táraka, lied to the difficult peaks of mount Meru, abandoning the field of battle. And the gods together with the Rishis went to the six-mouthed Kártikeya fur protection, and he, defending the gods, remained surrounded by them. When Indra heard that, he was troubled, considering that his kingdom was taken from him, and being jealous he went and made war upon Kártikeya.

Then the goddess distracted with anger and grief, said, " I have not obtained a son from you after all," and Śiva said to her; "An obstacle has arisen in this matter, because you neglected to worship Ganésa, the lord of obstacles; therefore adore him now in order that a child may be born to us of the fire." "When thus addressed by Śiva, the goddess worshipped Ganésa, and the fire became pregnant with that germ of Śiva. Then, bearing that embryo of Śiva, the fire shone even in the day as if the sun had entered into it. And then it discharged into the Ganges the germ difficult to bear, and the Ganges, by the order of Śiva, placed it in a sacrificial cavity on mount Meru. There that germ was watched by the Ganas, Śiva's attend and after a thousand years had developed it, it became a boy with six faces. Then, drinking milk with his six mouths from the breasts of the six Krittikás appointed by Gaurí to nurse him, the boy grew big in a few days. In the meanwhile, the king of the gods, overcome by the Asura Táraka, lied to the difficult peaks of mount Meru, abandoning the field of battle. And the gods together with the Rishis went to the six-mouthed Kártikeya fur protection, and he, defending the gods, remained surrounded by them. When Indra heard that, he was troubled, considering that his kingdom was taken from him, and being jealous he went and made war upon Kártikeya.