Aishah (RA) reported that the Messenger of Allah (peace and blessings of Allah be upon him) was given a sheep and he ordered for it's distribution.

When the servant would come back (after distributing it), Aishah would ask: “What did they say?”, he replied: They would supplicate:

“May Allah bless you.”

…Aishah would then say:

“and may Allah bless you.”

…we return their supplication in a similar way and our reward remains with us.

[For a male]
baarak-allaahu feek

[For a female]
baarak-allaahu feeki

[For a group of people]
baarak-allaahu feekum