It wasnarrated from Abu Sa'ib that he heard Abu Hurairah say:
"TheMessenger of Allah (ﷺ) said: 'Whoever performs a prayer in whichhedoes not recite Ummul Qur'an (the Mother of the Qur'an, i.e.,Al-Fatihah), it is deficient; not complete.'" I said: 'O AbuHurairah,sometimes I am behind the Imam. He pressed my forearm andsaid: 'OPersian! Recite it to yourself.'" (IT)
English reference (IT): Vol. 1, Book 5, Hadith 838 (IT)
Arabic reference (IT): Book 5, Hadith 887 (IT)