Ubaydullah bin Abi Yazid said: My father invited Sayyiduna Abdullah bin Umar (may Allah be pleased with them both). He came and sat down, and the food was placed. Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) put his hand toward the food and said: "Begin, saying 'Bismillah' (In the name of Allah)." Then Sayyiduna Abdullah (may Allah be pleased with him) withdrew his hand and said: "I am fasting."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14535