Is he sinful who does not believe one who swears by Allah while not praying?

A Muslim should not swear excessively by Allah, even if truthful, as it contradicts reverence and indicates heedlessness. Allah Almighty says: "And guard your oaths." As for a false oath, it is utterly forbidden, and the sin increases if it is for a worldly purpose. The Prophet, peace and blessings be upon him, said: "There are three whom Allah will not speak to, nor look at, and for them is a painful punishment: the one who reminds others of his favors, the one who lets his garment drag out of arrogance, and the one who sells his goods by a false oath." It is not obligatory to believe the one who swears, especially if he is a transgressor or known for neglecting his duties.