Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.114.

Safwan ibn ‘Abdallah reported, “I visited Abu Darda’s house in Syria. I did not find him there but Umm Darda was present at the house. She asked, ‘Do you intend to perform Hajj during this year?’ I replied, ‘Yes.’ She said, ‘Do supplicate Allah for us, for Allah’s Messenger, peace be upon him, used to say, “The supplication of a Muslim for his brother in his absence is accepted when he makes a supplication for blessings for his brother, and the commissioned Angel says, ‘Amen, May it be for you too!”” I went to the market and met Abu Darda and he narrated a similar report from Allah’ s Messenger, peace be upon him. (Reported by Muslim and Abu Daw’ud)

Abdallah ibn ‘Amr ibn al-‘As reported that the Prophet, peace be upon him, said, “The supplication that gets the quickest answer is the one made by one Muslim for another in his absence.” (Abu Daw’ud and Tirmizhi)

‘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 younger brother, do not forget me in your prayers.’ ‘Umar said, ‘It was a word more pleasing to me than the entire wealth of the world’.”