Is the prayer of one valid who intended to salute the guardian angels without intending to exit the prayer, then doubted and repeated the salutation and prostrated for forgetfulness?

The general principle is that the worshipper should intend to exit the prayer with the Taslim (definition) (salutation), just as they intend to begin it with the Takbir (proclamation of 'Allahu Akbar'). The evidence for this is the hadith (definition): "The key to prayer is purification, its prohibition (of worldly acts) is the Takbir, and its permissibility (of worldly acts) is the Taslim." However, if the worshipper forgets to make the intention at the time of Taslim, their prayer is still valid, because the intention made at the beginning of the prayer encompasses all its actions and utterances, and doubt after completing an act of worship has no effect.