What should someone who does not know the Qunut supplication do?

According to the Hanafis, it is obligatory (wajib) to say the Qunut takbir and recite a supplication in the third rak‘ah of the Witr prayer. Reciting the supplications known as “Allahumma inna nasta‘inuka” and “Allahumma iyyaka na‘budu” (Ibn Abi Shayba, al-Musannaf, 2/95, 6/89 [6893, 29708]; Tahawi, Sharh Ma‘ani al-Athar, 1/249 [1475]) is considered Sunnah. A person who does not know these supplications should strive to memorize them; however, until they are memorized, one may recite the supplication “Rabbana atina” or suffice by saying “Allahummaghfir li” three times (see Ibn Abidin, Radd al-Muhtar, 2/6-7; Bilmen, Ilmihal, 132, 158).

According to the Shafi‘i and Maliki schools, Qunut is performed after the bowing (ruku‘) in the second rak‘ah of the Fajr (morning) prayer. Performing Qunut in the morning prayer is Sunnah according to the Shafi‘is and recommended (mustahabb) according to the Malikis. A Hanafi who prays behind an imam from the Shafi‘i or Maliki school in the morning prayer may choose to join in the Qunut supplication or wait silently (Marghinani, al-Hidaya, 1/66).

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu