Authentic Supplication Number 205.

It is reported by way of Wahshiy that the companions of the Messenger of Allah  said: O Messenger of Allah! We eat; however we do not get full. He said: Perhaps you eat separately? They said: Yes. He  said: Then join together upon your food, and invoke he name of Allah: blessing would be bestowed in it for you.

Reported by Abu Dawood, and Ibn Majah. Al-Albani said that Ibn Hiban said it was sound. Al-Albani said, in Saheeh al-jamih al-sagheer wa ziyadatu (#142) that it was comely. He included this hadeeth in The Authentic of Good Sayings as #146.