Prophet Mohamed (S.A.W.) told Rabia to ask him for something - anything he wanted.

http://falaah.org/wp-content/uploads/2009/06/The-Eternal-Journey-Notes-Introduction.pdf

STORY - Young Sahaba named Rabia )بينة ( who assigned himself the job of serving the Prophet water. Keep in mind that he was 16 while the Prophet Mohamed (S.A.W.) was almost 61. Rabia stood by the Prophet’s door constantly to offer him water for wudu. Rabia would hear Prophet Mohamed saying “Subhana Rabbia Allah” in sujood at night. Prophet Mohamed (S.A.W.) came out of his room one day and he saw Rabia was standing with water for him. Prophet Mohamed (S.A.W.) told Rabia to ask him for something - anything he wanted. Rabia didn’t answer right away but asked the prophet for some time to think. When he thought it over, Rabia said that just as he is with the Prophet now, he wants to be with the Prophet in Jannah – he realized the meaning of Jannah. Prophet Mohamed (S.A.W.) said if Rabia wanted to be with him in Jannah, he should do lots of sujood, i.e. do lots of extra prayers

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