Hadith
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
Sahih · Salim al-Hilali
Muwatta Malik 431 ·Book 10, Hadith 4·Also available on sunnah.com·