أَخْبَرَنَا مُحَمَّدُ بْنُ عُبَيْدِ بْنِ مُحَمَّدٍ، قَالَ حَدَّثَنَا حَاتِمُ بْنُ إِسْمَاعِيلَ، عَنْ هِشَامِ بْنِ إِسْحَاقَ بْنِ عَبْدِ اللَّهِ بْنِ كِنَانَةَ، عَنْ أَبِيهِ، قَالَ سَأَلْتُ ابْنَ عَبَّاسٍ عَنْ صَلاَةِ، رَسُولِ اللَّهِ صلى الله عليه وسلم فِي الاِسْتِسْقَاءِ فَقَالَ خَرَجَ رَسُولُ اللَّهِ صلى الله عليه وسلم مُتَبَذِّلاً مُتَوَاضِعًا مُتَضَرِّعًا فَجَلَسَ عَلَى الْمِنْبَرِ فَلَمْ يَخْطُبْ خُطْبَتَكُمْ هَذِهِ وَلَكِنْ لَمْ يَزَلْ فِي الدُّعَاءِ وَالتَّضَرُّعِ وَالتَّكْبِيرِ وَصَلَّى رَكْعَتَيْنِ كَمَا كَانَ يُصَلِّي فِي الْعِيدَيْنِ ‏.‏

It was narrated from Hisham binIshaq bin Abdullah bin Kinanah that: His father said: "I asked Ibn 'Abbas how the Messenger of Allah (PBUH) prayed for rain. He said: 'The Messenger of Allah (PBUH) went out (dressed) in a state of humility, beseeching and humble. He sat on the minbar but he did not deliver a Khutbah like this Khutbah of yours, rather he kept supplicating, beseeching and saying the takbir, and he prayed two rak'ahs as he used to do during the two 'Eids.'" Translation Not Available

Book reference: Sunan An Nisai Book 17 Hadith 1508
Web reference: Sunan An Nisai Volume 2 Book 17 Hadith 1509