الكافي- ط الاسلامية, volume 2, page 517

2. It is narrated from him (narrator of the Hadith above) from al-Fudayl ibn ‘Abd al-Wahhab from Ishaq ibn ‘Abd Allah from ‘Ubayd Allah ibn al-Walid al- Wassafi in a marfu ’ manner who has said the following: ‘‘The Messenger of Allah has said, ‘Whoever (sincerely) says, ‘No one deserves to be worshipped except Allah’, a tree is planted for him in paradise, from red ruby that grows out of white musk, sweeter than honey, whiter than snow, of a fragrance better than musk, in which there is something like the breast of virgins, protruding from behind seventy dresses.’ ‘‘The Messenger of Allah has said, ‘The best worship is the words: ‘No one deserves to be worshipped except Allah.’ ‘‘He also has said, ‘The best worship is pleading before Allah for forgiveness, due to the words of Allah, the Most Majestic, the Most Holy, in His book: ‘(you must) know that no one deserves to be worshipped except Allah. Ask forgiveness for your sins. .. .’”(47:19)

Translation: Muhammad Sarwar·Thaqalayn link·The Arabic above is the original.