Silsilat al-Ahadith al-Sahihah

Sayyiduna Anas bin Malik (may Allah be pleased with him) says: In the time of the Messenger of Allah (peace and blessings be upon him), when the muezzin finished the call to prayer for Maghrib, the distinguished Companions would rush towards the pillars and (using them as a barrier) would perform two rak‘ahs before Maghrib. When the Prophet (peace and blessings be upon him) would arrive, they would be praying. People would perform these two rak‘ahs so frequently that a stranger would feel as if the prayer had already been offered (and people were performing the subsequent Sunnah prayers). There was only a short interval between the adhan and the iqamah.

It is narrated from Sayyiduna Abdullah bin Zubair (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "There is no obligatory prayer except that before it there are (at least) two units (rak‘ahs) of voluntary prayer."