This representation of the standing, newborn Sakyamuni pointing to the sky with his left index finger, and to the ground with his right index finger is described in Asvaghosa's Life of Buddha translated by Samuel Beal, Sacred Books of the East, vol. XIX, Oxford, 1883, p. 226:
'Upright and firm and unconfused in mind, he deliberately took seven steps...he spoke thus with the fullest assurance: "This birth is in the condition of a Buddha; after this I have done with renewed birth: now only am I born this once, for the purpose of saving all the world'.