Reference : Hisn al-Muslim 200

أَحَبَّـكَ الّذي أَحْبَبْـتَني لَه
Aḥabbaka ‘lladhī aḥbabtanī lah.

May He for Whose sake you love me, love you.

Reference:
Abu Dawud 4/333. Al-Albani graded it good in Sahih Abu Dawud 3/965.

Chapter: To one who pronounces his love for you, for Allah’s sake