Narrated by as-Saduq in his “al-Khisal” (p 358):

Translation:

Muhammad ibn Ali Majiluyih – may God be pleased with him – narrated that Muhammad ibn Yahya al-Attar quoted Muhammad ibn Ahmad, on the authority of Ahmad ibn Hilal , on the authority of Isa ibn Abdullah al-Hashemi, on the authority of his father, on the authority of his forefathers (alaihuma salam) that God’s Prophet (sallalahu alaihi wa sallam) said, “Gabriel was sent to me by the Honorable the Exalted God and told me, ‘God has ordained that you recite the Quran in one letter only.’ I said, ‘O God! Please be more lenient with my nation.’ Then Gabriel said, ‘The Honorable the Exalted God ordained that you recite the Quran in seven letters.’”