Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.144.
Section : Asking the Traveller “Pray for Us”.
‘Umar ibn al-Khattab reports, “I sought permission of the Prophet, peace be upon him, to perform ‘umrah. He gave me permission and said, ‘My brother, do not forget me in your prayers.” ‘Umar reported further, “This was a word more pleasing to me than the entire wealth of the world.” (Reported by Abu Daw’ud and Tirmizhi, who holds it to be a sound hadith)