Ata (may Allah have mercy on him) said that Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) was invited to a meal while he was busy managing the water-supply (siqayah). He said to the people: "Go to your brother," or: "Accept your brother's invitation, convey my greeting of peace (salam) to him, and tell him that I am busy."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14540