أَحَبَّكَ الَّذِي أَحْبَبْتَنِي لَهُ

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

'Ahabbakal-lathee 'ahbabtanee lahu.

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