To what extent is the hadith authentic: "Whoever wishes that Allah Almighty save him from the nineteen Zabaniyah, let him recite 'Bismillah Al-Rahman Al-Rahim,' for it has nineteen letters, so Allah will make each letter of them a shield from one of them"?

This narration is a saying of Ibn Mas'ud, may Allah be pleased with him, and not a saying of the Prophet, may Allah’s prayers and peace be upon him. Its chain of transmission to Ibn Mas'ud is authentic, via Wakee' from Al-A'mash from Abu Wa'il. The narration of Al-A'mash from Abu Wa'il is acceptable and considered continuous, because he narrated extensively from him, even if he was a mudallis (one who practices tadlis - concealing defects in the chain of narration). However, there remains the question of the condition of those below Wakee', as Al-Qurtubi and Ibn Katheer did not mention the complete chain of transmission. But Al-Suyuti attributed the narration to Wakee', and Wakee' has a famous Tafsir (definition) (exegesis). If this is established, then the chain is authentic, and the narration from Ibn Mas'ud becomes valid thereby.