Muwatta Malik, Book 15, Hadith 15 (USC-MSA)

Yahya related to me from Malik from Ibn Shihab from al-Araj that Umar ibn al Khattab recited Surat an-Najm (Sura 53) and prostrated in it, and then got up and recited another sura.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنِ الأَعْرَجِ، ‏.‏ أَنَّ عُمَرَ بْنَ الْخَطَّابِ، قَرَأَ بِالنَّجْمِ إِذَا هَوَى فَسَجَدَ فِيهَا ثُمَّ قَامَ فَقَرَأَ بِسُورَةٍ أُخْرَى ‏.‏

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also referenced under: Book 15, Hadith 487 (In-Book)