Sahih al-Bukhari 6028
Sahih al-Bukhari · Book 78: Good Manners and Form (Al-Adab) · "Whosoever intercedes for a good cause will have the reward thereof..."
Narrated Abu Musa:
حَدَّثَنَا مُحَمَّدُ بْنُ الْعَلاَءِ ، حَدَّثَنَا أَبُو أُسَامَةَ ، عَنْ بُرَيْدٍ ، عَنْ أَبِي بُرْدَةَ ، عَنْ أَبِي مُوسَى ، عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ كَانَ إِذَا أَتَاهُ السَّائِلُ أَوْ صَاحِبُ الْحَاجَةِ قَالَ " اشْفَعُوا فَلْتُؤْجَرُوا، وَلْيَقْضِ اللَّهُ عَلَى لِسَانِ رَسُولِهِ مَا شَاءَ " .
Whenever a beggar or a person in need came to the Prophet, the Prophet would say "Help and recommend him and you will receive the reward for it, and Allah will bring about what he will through His Prophet's tongue