(110) بَابُ مَنْ صُنِعَ إِلَيْهِ مَعْرُوفٌ فَلْيُكَافِئْهُ
Ibn 'Umar reported that the Messenger of Allah, may Allah blesshim and grant him peace, said, "Anyone who seeks refuge in Allah will findrefuge with Him. Anyone who asks from Allah will be receive. Anyone whodoes a favour should repay it. If you do not find anything, then make supplicationfor the doer of the favour so that he knows that you have repaid him."
Reference: Al-Adab Al-Mufrad 216
In-book reference: Book 10, Hadith 5
English translation: Book 10, Hadith 216