It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) forbade pointing at the rain.
It has also been narrated through another, weak chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6474