Al-Kāfi - The Book of Prayer

Chapter: Salah (prayer) for Rain 4ـ وَفِي رِوَايَةِ ابْنِ الْ٠ُغِيرَةِ قَالَ يُكَبِّرُ فِي صَلاَةِ الاسْتِسْقَاءِ كَ٠َا يُكَبِّرُ فِي الْعِيدَيْنِ فِي الأولَى سَبْعاً وَفِي الثَّانِيَةِ خَ٠ْساً وَيُصَلِّي قَبْلَ الْخُطْبَةِ وَيَجْهَرُ بِالْقِرَاءَةِ وَيَسْتَسْقِي وَهُوَ قَاعِدٌ. 4.

It is in the narration of ibn al-Mughirah: that Takbir (Allah is great beyond description) in Salah (prayer) for rain is said in the same way as on the day of ‘Id, seven times in the first Rak‘at and five times in the second Rak‘at. He (the Imam) performs Salah (prayer) before the sermon; recites aloud and pleads for rain when he is sitting.

(Ahadith 3-4 and the following chapter are best by fatwa.) سكت عنه - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (5/438))