The Messenger of Allaah (peace and blessings of Allaah be upon him) said:

“Whoever has a favour done for him and says to the one who did it, ‘Jazakal lahu khayran,’ has done enough to thank him.”

Narrated by al-Tirmidhi (1958) and by al-Nasaa’i in al-Sunan al-Kubra (6/53); classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.