Yahya related to me from Malik from AbunNadr, the mawla of Umar ibn Ubaydullah, that Abu Salama ibn Abd ar-Rahman asked him, "Didnt I see your master sit down before praying after he had entered the mosque?" Abun-Nadr said, "By that he meant Umar ibn Ubaydullah, and he was finding fault with him for sitting down before praying after he had come into the mosque." Yahya said that Malik said, "It is good to do that but not obligatory."