It is narrated from Sayyiduna Mujahid (may Allah have mercy on him) that the son of Sayyiduna Ibn Umar (may Allah be pleased with them both) — Abu Bakr or Umar — sneezed and said: "Aab." Sayyiduna Ibn Umar (may Allah be pleased with them both) said: "What is 'Aab'? 'Aab' is the name of one of the devils, which he has placed between sneezing and «الحمد للہ»."
Hadith Referenceالادب المفرد / كتاب العطاس والتثاؤب / 937
Benefits and Issues: The point is that after sneezing, the sound "aab" or, according to some narrations, "aash" or "ashhab" that is uttered should be avoided, because this is the name of Shaytan, and he desires that his name be mentioned before the name of Allah. Shaykh al-Albani rahimahullah has considered the narration with "aash" to be more authentic than the one with "aab."
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 937