'Aaishah (radhi-yAllaahu 'anhaa) reported that the Messenger of Allaah (sal-Allaahu 'alayhe wa sallam) was given a sheep and he ordered for it's distribution. When the servant would come back [from distributing it] , 'Aaishah would ask: "What did they say?", he replied: "They would supplicate:

بَارَكَ اللَّهُ فِيكُمْ

"May Allaah bless you all.'

barakAllahu feekum

...'Aaishah would then say:

وَ فِيهِم بَارَكَ اللَّهُ

"and may Allaah bless them."

wa feehim barakAllahu

So we return their supplication in a similar way and our reward remains with us:

وَفِيكَ بَارَكَ اللَّهُ

"and May Allah bless you.'

wa feeka barakAllahu

Ibn As-Sunni (Page 138 , Hadith no. 278).Also see, Ibnul Qaiyyum's Al Wabilus Sayyer Page 308. Tahkeek, Bashir Mamud Uyyun.