The Messenger of ALLAH (peace and blessings be upon him) said, “If good is done to someone and then they say “Jazak ALLAHu khayran” to the one who did the good, they have indeed praised them well.” [Tirmidhi]
What should one reply if someone says JazakALLAHu khayran to him?
And you too, May ALLAH reward to with Khayr. The Hadeeth is narrated by Usayd Ibn Hudair (R.A),
Usayd ibn Hadayr says … I said :
O Messenger of ALLAH [JazakALLAHu Khayrn]
The Prophet (S.A.W) said :
And you too, May ALLAH reward you too with Khayr.
Shaykh al-Albaany has said that the Hadeeh is Saheeh. (bi-kathrat-il-turuq)
[al-Saheeha 3096, al-Ta'leeqaatul hisaan al Saheeh ibn Hibbaan 6231]