Abu Dawud Book 2, Hadith Number 1493.
Chapter : Supplications.
Narated By Umar ibn al-Khattab : I sought permission of the Prophet (PBUH) to perform umrah. He gave me permission and said: My younger brother, do not forget me in your supplication.
He (Umar) said: He told me a word that pleased me so much so that I would not have been pleased if I were given the whole world.
The narrator Shu’bah said: I then met Asim at Medina. He narrated to me this tradition and reported the wordings: “My younger brother, share me in your supplication.”