Mishkat al-Masabih 4 Prayer كتاب الصلاة 908

وَعَنْ أَنَسٍ أَنَّ عُمَرَ بْنَ الْخَطَّابِ كَانَ إِذْ قحطوا استسقى بالبعاس بْنِ عَبْدِ الْمُطَّلِبِ فَقَالَ: اللَّهُمَّ إِنَّا كُنَّا نَتَوَسَّلُ إِلَيْكَ بِنَبِيِّنَا فَتَسْقِينَا وَإِنَّا نَتَوَسَّلُ إِلَيْكَ بِعَمِّ نَبِيِّنَا فَاسْقِنَا. قَالَ: فَيُسْقَوْنَ. رَوَاهُ الْبُخَارِيُّ

Anas said that when they experienced a drought ‘Umar b. al-Khattab prayed for rain by virtue of al-‘Abbas b. ‘Abd al-Muttalib, and said, “O God, we used to supplicate Thee by virtue of our prophet and Thou didst give us water. We are now supplicating Thee by virtue of our prophet’s uncle, so give us water.” He said that they were then given water. Bukhari transmitted it.