The Prophet (peace and blessings be upon him) said:
"When you are given something without having asked for it, eat from it and give (the remainder) in charity."
Hadith Referenceصحیح الجامع الصغیر / حرف الألف / 359
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[م د ن] عن عمر. الإرواء ٨٦٢، المشكاة ١٨٥٤.