Muwatta Malik, Book 15, Hadith 509 (In-Book)

Yahya related to me from Malik that Abdullah ibn Dinar said, "Abdullah ibn Umar saw me when I was making dua and I was pointing with two fingers, one from each hand, and he forbade me."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، قَالَ رَآنِي عَبْدُ اللَّهِ بْنُ عُمَرَ وَأَنَا أَدْعُو، وَأُشِيرُ، بِأَصْبُعَيْنِ أَصْبُعٍ مِنْ كُلِّ يَدٍ فَنَهَانِي ‏.‏

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also referenced under: Book 15, Hadith 37 (USC-MSA)