Muwatta Malik 628
Zakat
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ لاَ يُخْرِجُ فِي زَكَاةِ الْفِطْرِ إِلاَّ التَّمْرَ إِلاَّ مَرَّةً وَاحِدَةً فَإِنَّهُ أَخْرَجَ شَعِيرًا .
Yahya related to me from Malik from Nafi that Abdullah ibn Umar would always pay the zakat al-fitr in dates, except once, when he paid it in barley
Grading: Salim al-Hilali: Mauquf Sahih
Text from the public-domain hadith-api dataset. Numbering follows that dataset and can differ by a few from some printed editions — check the wording, not only the number.