«كان إذا دخل قال:» هل عندكم طعام [؟ فإذا قيل لا قال: ] إني صائم"
It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) entered a house, he would say: Do you have any food? If he was told no, he would say: Then I am fasting."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4719