When someone tells you he/she loves you for the sake of Allah, you should respond with the following dua.
“May He for Whose sake you love me, love you”
أَحَبَّـكَ الّذي أَحْبَبْـتَني لَه
‘Ahabbakal-lathee ‘ahbabtanee lahu
Reference: Abu Dawud 4/333. Al-Albani graded it good in Sahih Abu Dawud 3/965.
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