It is correct that as-Sattaar is not mentioned either in Qur’an or hadith, instead as-Sitteer is one of the names of Allaah swt mentioned in hadith that is narrated by Abu Dawood (4012) that the Prophet Muhammad (blessings and peace of Allaah be upon him) said: “Allaah is characterized by modesty (al-Hayye) and concealment (as-Sitteer), and He loves modesty and concealment. When one of you washes himself, let him conceal himself.” It was classed as saheeh by Shaikh Nasir uddin al-Albaani in Saheeh al-Jaami’.
Shaikh Imam Abd al-Wahhaab (1703 – 1792) in his book ‘Kitaab At-Tawheed’ (Ch. 49) writes that it is not permissible to derive names for Allah from the verbs used in the Qur'an to describe His Actions: Thus, we may not refer to Him as Al-Mun'im (the Provider of Blessings), even though the Qur'an speaks of His Ni'am (Blessings), nor is it allowed for us to call Him As-Sattaar (the Veiler) even though He has been described as As-Satteer (derived from the same verb) in an authentic Hadith; and whoever does so is in error.
According to Sheikh Muhammad ibn al-Uthaymeen at-Tamimi (1925– 2001) the names of Allaah can only be known through Qura’n and sound texts of hadeeth, with no room for ijtihad, so He is not to be called by anything except that by which he called Himself or that His Messenger (blessings and peace of Allaah be upon him) called Him.
However, some scholars from South Asian countries are of the opinion that the root word of ‘as-Sattaar’ is ‘Satr’ or ‘Sitr’ meaning veil, cover, conceal, curtain, shelter etc. and ‘Sitteer’ means one who covers or conceals the bad deeds of some one and ‘as-Sitteer’ means the ‘Lord’ Who is best of ‘Coverer’ and Who conceals the bad deeds of mankind, thus ‘as-Sattaar’ is synonymous to ‘as-Sitteer’. Also as ‘al-Ghaffaar’ or ‘al-Ghaffoor’ relates to ‘al-Ghafir’, (40:3) and ‘al’Qahhaar’ (39:4) relates to ‘al-Qaahir’ (6:18), ‘as-Sattaar relates to ‘as-Sitteer’.
And Allaah knows the best.