مُوَطَّأُ مَالِكٍ
Muwatta Malik 434
The Two 'Ids
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ أَبَا بَكْرٍ، وَعُمَرَ، كَانَا يَفْعَلاَنِ ذَلِكَ .
Yahya related to me from Malik that he had heard that Abu Bakr and Umar used to do that.
Arabic/English book reference: Book 10, Hadith 4