Start with this Du'a
Replying to "I love you for the sake of Allah (SWT)"
أَحَبَّكَ الَّذِي أَحْبَبْتَنِي لَهُ
Ahabbaka-lladhi ahbabtani lah.
“May the One for whose sake you love me, love you.”
Sunan Abi Dawud 5125 (hasan)
The reply to someone who tells you they love you for the sake of Allah (SWT).