It was narrated to us by Moammed bin Ibrāhīm bin ‘Isāq رحمه الله who said, it was narrated to us by Amed bin Moammed al-Kūfi, from ʿAlī bin al-ussain bin ʿAlī bin fadhāl, from his father:

From Abī al-Ḥassan,  ʿAlī bin Mūsa al-Ridha عليه السلام who said: Whoever fasts the first day of [the month of] Rajab – yearning for the reward from Allāh, Mighty and Majestic – Paradise becomes obligatory for him.
Whoever fasts a day in the middle [of the month of Rajab], intercession [will be accepted] on behalf of as many people in the Rabīʿah and Modhar [tribes].
Whoever fasts a day from the end [of the month of Rajab], Allāh, Mighty and Majestic, renders him to be of the Kings of Paradise – and will have intercession [accepted] on behalf of his father, mother, son, daughter, brother, sister, paternal uncle, paternal aunt, maternal uncle, maternal aunt, friends and neighbours; even if within them are those whom the fire has been made obligatory upon them.

حدثنا محمد بن إبراهيم بن إسحاق (رحمه الله)، قال: حدثنا أحمد بن محمد الكوفي، عن علي بن الحسن بن علي بن فضال، عن أبيه، عن أبي الحسن علي ابن موسى الرضا (عليه السلام) قال: من صام أول يوم من رجب رغبة في ثواب الله عز وجل وجبت له الجنة، ومن صام يوما في وسطه شفع في مثل ربيعة ومضر، ومن صام يوما في آخره جعله الله عز وجل من ملوك الجنة، وشفعه في أبيه وأمه، وابنه وابنته، وأخيه وأخته، وعمه وعمته، وخاله وخالته، ومعارفه وجيرانه، وإن كان فيهم مستوجب للنار