Al-Kāfi - Volume 2 > Asking Favors from Allah for the Holy Prophet, Muhammad and his Family, recipient of divine supreme covenant
Hadith #10

10ـ مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنْ عَلِيِّ بْنِ الْحَكَمِ وَعَبْدِ الرَّحْمَنِ بْنِ أَبِي نَجْرَانَ جَمِيعاً عَنْ صَفْوَانَ الْجَمَّالِ عَنْ أَبِي عَبْدِ الله (عَلَيهِ السَّلام) قَالَ كُلُّ دُعَاءٍ يُدْعَى الله عَزَّ وَجَلَّ بِهِ مَحْجُوبٌ عَنِ السَّمَاءِ حَتَّى يُصَلَّى عَلَى مُحَمَّدٍ وَآلِ مُحَمَّدٍ.

10. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad from Ali ibn al-Hakam and ‘Abd al-Rahman ibn abu Najran all from Safwan al-Jammal from abu ‘Abd Allah (a.s.) who has said the following: “Every prayer to Allah, the Most Majestic, the Most Holy, remains on hold and barred from heavens until he asks from Allah favors (in the form of al-Salat) for the Holy Prophet and his family.”