Jami` at-Tirmidhi 558
It is narrated from Hisham bin Ishaq - and he was from Ibn Abdullah bin Kinanah - from his father who said:
"Al-Walid bin Uqbah, the governor of Al-Madinah, sent me to ask Ibn Abbas about how the Messenger of Allah would perform Salat Al-Istisqa. I came to him and he said: 'The Messenger of Allah would go out in modest dress, humbly, imploring, until he reached the Musalla. He would not give this Khutbah of yours, rather, he would continue supplication and imploring saying the Takbir, and pray two Rak'ah, just as he would pray for the Eid.'"
حَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا حَاتِمُ بْنُ إِسْمَاعِيلَ، عَنْ هِشَامِ بْنِ إِسْحَاقَ، وَهُوَ ابْنُ عَبْدِ اللَّهِ بْنِ كِنَانَةَ عَنْ أَبِيهِ، قَالَ أَرْسَلَنِي الْوَلِيدُ بْنُ عُقْبَةَ وَهُوَ أَمِيرُ الْمَدِينَةِ إِلَى ابْنِ عَبَّاسٍ أَسْأَلُهُ عَنِ اسْتِسْقَاءِ، رَسُولِ اللَّهِ صلى الله عليه وسلم فَأَتَيْتُهُ فَقَالَ إِنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم خَرَجَ مُتَبَذِّلاً مُتَوَاضِعًا مُتَضَرِّعًا حَتَّى أَتَى الْمُصَلَّى فَلَمْ يَخْطُبْ خُطْبَتَكُمْ هَذِهِ وَلَكِنْ لَمْ يَزَلْ فِي الدُّعَاءِ وَالتَّضَرُّعِ وَالتَّكْبِيرِ وَصَلَّى رَكْعَتَيْنِ كَمَا كَانَ يُصَلِّي فِي الْعِيدِ ‏.‏ قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ ‏.‏