Yahya related to me from Malik from Abu'nNadr, the mawla of Umaribn Ubaydullah, that Abu Salama ibn Abd ar-Rahman asked him, "Didn't Isee your master sit down before praying after he had entered themosque?"Abu'n-Nadr said, "By that he meant Umar ibnUbaydullah, and he was finding fault with him for sitting down beforepraying after he had come into the mosque."Yahya said thatMalik said, "It is good to do that but not obligatory."
USC-MSA web (English) reference: Book 9, Hadith 61
Arabic reference: Book 9, Hadith 392