Muwatta Malik, Book 10, Hadith 4 (USC-MSA) ⓘ
Yahya related to me from Malik that he had heard that Abu Bakr and Umar used to do that.
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ أَبَا بَكْرٍ، وَعُمَرَ، كَانَا يَفْعَلاَنِ ذَلِكَ .
also referenced under: Book 10, Hadith 434 (In-Book)