Is it permissible for a girl to say to someone who proposed to her and was then rejected, "I love you for the sake of Allah and I ask Allah to unite me with you in Paradise"? And what is the evidence for that?

Marriage is the ideal solution for those who love each other, as the Prophet, peace and blessings be upon him, said: "We have not seen for two who love each other anything better than marriage." If marriage is not feasible, then both parties should disengage from this attachment and strive for chastity through marriage, occupying their time with what benefits them in religious and worldly matters. There is no objection to the woman supplicating that Allah unite them in Paradise, but corresponding with her and informing her of love for Allah's sake is not appropriate due to the increased attachment and temptation that may result from it.