Yahya related to me from Malik from Amir ibn Abdullah ibn az-Zubayr from Amr ibn Sulaymaz-Zuraqi from Abu Qatadaal-Ansari that theMessenger of Allah, may Allah bless him and grant him peace, said,"When you enter the mosque, you should pray two rakas before you sitdown."
USC-MSA web (English) reference: Book 9, Hadith 60
Arabic reference: Book 9, Hadith 391