Fortress of the Muslim (Hisn al-Muslim)
حصن المسلم
(89) Chapter: To one who pronounces his love for you, for Allah’s sake
(89) الدعاء لمن قال إني أحبك في الله
Hisn al-Muslim 200 
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.
أَحَبَّـكَ الّذي أَحْبَبْـتَني لَه
Reference : Hisn al-Muslim 200