The usual form of the name is a derivative — e.g., J.vi.212; he is often called Maghavā Sujampati — e.g., J.iii.146; iv.403; v.137, 139; vi.102, 481, 573; or Maghavā Sakko — e.g., J.v.141; see also Mtu.i.165, 167 (sahasranetro Maghavān va sobhase) and Mtu.iii.366 (Sakro āha: Maghavān ti me āhu syaloke).
His story is given in the Kulāvaka Jātaka. For a slightly different version see DhA.i.264 ﬀ. Because of his birth as Magha, Sakka came to be known as Maghavā. Maghavā was, perhaps, not the personal name of any particular Sakka, but a title of all kings of Tāvatiṃsā, because the Sakka who was the real Magha is identified with the Bodhisatta (J.i.207), while the Buddha says (S.i.231; DhA.i.264) that the Sakka, who visited him, and whose conversation is recorded in the Sakkapañha Sutta, was also known as Maghavā. The title probably originated from the time when Magha became Sakka.
The Saṃyuttanikāya Commentary (SA.i.267; this is supported by the story as given in DA.iii.710 ﬀ. and DhA.i.264 ﬀ., where no mention is made of the Bodhisatta), however, says that Magha was not the Bodhisatta, but that his life was like that of a Bodhisatta (Bodhisattacariyā viyassa cariyā ahosi); in which case the name Maghavā belongs only to the present Sakka. Magha took upon himself seven vows (vatapadāni), which brought him birth as Sakka: to maintain his parents, to revere his elders, to use gentle language, to utter no slander, to be free from avarice, to practise generosity and open handed liberality and kindness, to speak the truth, to be free from anger (S.i.227 f; SA.i.267).
For this and other titles of Sakka, see Sakka.