Muwatta Malik, Book 20, Hadith 119 (USC-MSA)

Yahya related to me from Malik that Abu'z Zubayr al-Makki said, "I saw Abdullah ibn Abbas doing tawaf after asr. Then he went into his room and I do not know what he did."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ أَبِي الزُّبَيْرِ الْمَكِّيِّ، أَنَّهُ قَالَ رَأَيْتُ عَبْدَ اللَّهِ بْنَ عَبَّاسٍ يَطُوفُ بَعْدَ صَلاَةِ الْعَصْرِ ثُمَّ يَدْخُلُ حُجْرَتَهُ فَلاَ أَدْرِي مَا يَصْنَعُ ‏.‏

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also referenced under: Book 20, Hadith 822 (In-Book)