Assalaamu alaykum. Is it haram to say to someone that their supplication was not answered although it was?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

If what is meant is that someone knows that someone else’s supplication was answered without the supplicater knowing it and he told him that his supplication was not answered, then this is a lie, and it is informing about something contrary to its reality while knowing that and deliberately doing so.

It is well-known Islamically that lying is forbidden, and it is considered a major sin that drags its doer to Hellfire – we seek refuge in Allah from that – as in the hadeeth in which the Prophet said, “…And lying leads to vice (wickedness, immorality, evil-doing), and vice leads to Hellfire, and a man continues to lie until he is written with Allah as a liar." [Al-Bukhari and Muslim]

Moreover, lying is one of the characteristics of the hypocrites, the Prophet said, “The signs of a hypocrite are three: whenever he speaks, he lies; whenever he promises, he breaks his promise; and whenever he is entrusted, he deceives." [Al-Bukhari and Muslim]

Allah knows best.