Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) narrates: The Noble Prophet (peace and blessings be upon him) said: "When any one of you comes (to the mosque) and the Imam is delivering the sermon or has come out for the sermon, he should perform two rak‘ahs of prayer."
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1590