As-Sanuusiyy in his book ˆUmdatu ‘Ahli-t-Tawfiiq says, “and it is impossible that Aļļaah should lie,

As-Sanuusiyy in his book ˆUmdatu ‘Ahli-t-Tawfiiq says, “and it is impossible that Aļļaah should lie, since His Speech agrees with His Knowledge…. (P.245).” In explaining why it is impossible that Aļļaah could lie he says, “Third, it has been established that Aļļaah is attributed with complete perfection, and truthfulness is an attribute of complete perfection which’s opposite is a flaw, and it is impossible that Aļļaah should be attributed with a flaw, so He must be truthful.(P. 248))”

So As-Sanuusiyy says that lying is impossible for Aļļaah in the mind’s eye, unlike some people claim. For the record, As-Sanuusiyy mentions in the explanation of Aş-Şugħraa, “It is impossible that Aļļaah could lie, because His Speech/Kalaam must agree with His Knowledge, and speech in agreement with knowledge cannot be but truthful (Sħarĥu ‘Ummu-l-Baraahiin, 280)⁠.”

See also:
–Refuting the Accusation that Asharis Consider it Rationally Possible for Allah to Lie
–Someone asked: The idea that it is not absolutely impossible for Aļļaah to lie is mentioned in some books attributed to famous scholars. Can we seriously consider calling such illustrious `ulema who were masters of `aqida to be kufar and those who deny their kufr themselves kufar?

References:
ˆUmdatu ‘Ahli-t-Tawfiiq. As-Sanuusiyy, Muĥammad ibn Yuusuf. Egypt: Jariidatu-l-Islaam, 1316. <http://ia311311.us.archive.org/1/items/SanousiaKoubra/SonossiyaKoubra.pdf&gt;.

Ĥaasħiyatu-d-Dusuuqiyy ˆalaa Ummi-l-Baraahiin wa Sħarĥuhaa. —. Beirut, Lebanon: Al-Maktabah Al-ˆAşriyyah, 1426.