Is one who says, "If you recite this supplication" – and he mentions a supplication not from the Sunnah – "such and such will happen to you, by Allah's Will," considered to be speaking without knowledge or fabricating a lie against Allah?

It is permissible for a Muslim to choose any supplication (du'a) that he likes, due to the generality of the Prophet's (peace be upon him) saying: "Let each of you choose the supplication most pleasing to him, and then supplicate to Allah, the Mighty and Majestic." As for linking reward to a specific ذكر (remembrance) without evidence, it is a lie and a fabrication, because the determination of reward and punishment is exclusively the prerogative of the Lawgiver alone, and speaking about Allah without knowledge is a grave sin.