Muwatta Malik, Book 15, Hadith 47 (USC-MSA)

Yahya related to me from Malik from Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, said, "You should not intend to pray either at sunrise or sunset."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ لاَ يَتَحَرَّ أَحَدُكُمْ فَيُصَلِّيَ عِنْدَ طُلُوعِ الشَّمْسِ وَلاَ عِنْدَ غُرُوبِهَا ‏" ‏ ‏.‏

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also referenced under: Book 15, Hadith 519 (In-Book)