Muwatta Malik #393
Shortening the Prayer
Yahya related to me from Malik from Abu'nNadr, the mawla of Umar
ibn Ubaydullah, that Abu Salama ibn Abd ar-Rahman asked him, "Didn't I
see your master sit down before praying after he had entered the
mosque?"
Abu'n-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."
About Muwatta Malik
This hadith is #393 from Muwatta Malik, in the Book of Shortening the Prayer. Muwatta Malik contains 1,985 hadiths across 61 books.