Adbullaah Ibn Sarjis (radhi-yAllaahu 'anhu) said: " I went to see The Prophet (sal-Allaahu 'alayhe wa sallam) and ate from his food and then said to him:

غَفَرَ اللَّهُ لَكَ يَا رَسُولَ للَّهِ

"May Allaah forgive you, O Messenger of Allaah.'

GafarAllahu laka wa Rasoolallahi

...He replied:

وَلَكَ

And you.

Walaka

Ahmad 5/82, and An-Nasa'i in 'Amalul-Yawm wal-Laylah p. 218, with checking by Dr. Farooq Hamadah.