Sunan Abi Dawud, Hadith 4216

Nafi' said that Ibn 'Umar used to wear his signet-ring on his left hand.

حَدَّثَنَا هَنَّادٌ، عَنْ عَبْدَةَ، عَنْ عُبَيْدِ اللَّهِ، عَنْ نَافِعٍ، أَنَّ ابْنَ عُمَرَ، كَانَ يَلْبَسُ خَاتَمَهُ فِي يَدِهِ الْيُسْرَى ‏.‏

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also referenced under: Hadith 4228 (DarusSalam)