Malik bin Numayr al-Khuza'i (may Allah have mercy on him) said that his father (may Allah be pleased with him) narrated to him that he saw the Messenger of Allah (peace and blessings be upon him) sitting in the prayer; he (peace and blessings be upon him) had placed his right forearm on his right thigh and raised his index finger, and he had bent it slightly, while he was supplicating.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2785