The Book of Prayers (Kitab Al-Salat)

Muslim :: Book 4 : Hadith 1540

Abu Qatada (a Companion of the Prophet) reported Allah's Messenger (may peace be upon him) as saying: When any one of you enters the mosque, he should observe two rak'ahs (of Nafl prayer) before sitting.

Source materials are from the University of Southern California MSA site
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